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A 


COMPEND  OF  GEOLOGY 


BY 


JOSEPH  LE  CO]N"TE 


PROFESSOR   OF    GEOLOGY    AND    NATURAL    HISTORY    IN   THE    UNIVERSITY 
OF   CALIFORNIA;   AUTHOR   OF  **  ELEMENTS   OF   GEOLOGY,"  ETC. 


1 J  J   > » 
»        •    • 


KEW  YORK    .    CINCINNATI    •     CHICAGO 

AMERICAN    BOOK    COMPANY 


COPTBIGHT,   1884,   BT 

D.  APPLETON  &  CO. 

Copyright,  1898,  by 
AMERICAN   BOOK   COMPANY 

Copyright,  1912,  by 
CAROLINE   E.   LE   CONTE 


iE  CONTE.  QEOU 
E.  P.     2 


PREFACE. 


In  preparing  this  little  work  for  the  schools  I  have 
kept  constantly  in  view  two  ends  :  1.  I  have  tried  to  make 
a  book  which  shall  interest  the  pupil,  and  at  the  same 
time  convey  real  scientific  knowledge.  2.  I  have  tried, 
as  far  as  possible,  to  awaken  the  faculty  and  cultivate  the 
habit  of  observation,  by  directing  the  attention  of  the 
pupil  to  geological  phenomena  occurring  and  geological 
agencies  at  work  noio  on  every  side,  and  in  the  most 
familiar  tlmigs.  By  the  former  I  hope  to  awaken  a  true 
scientific  appetite ;  by  the  latter,  to  cultivate  the  habits 
necessary  tb  satisfy  that  appetite. 

Joseph  Le  Contb. 

Berkeley,  California,  September^  1884. 


PREFACE   TO  THE   REVISED  EDITION. 

Although  a  work  so  elementary  as  this — embodying 
only  the  most  general  principles  of  geology — does  not 
require  so  frequent  revision  as  a  more  advanced  work,  yet 
geology  is  so  rapidly  advancing  a  science  that  even  gen- 
eral statements  must,  from  time  to  time,  be  modified. 
Especially  is  it  necessary  that  new  and  better  illustrative 
figures  should  be  used. 


4  PREFACE   TO   THE  REVISED  EDITION. 

In  this  revised  edition  I  have  not  changed  the  general 
plan  of  the  work  as  already  explained  in  the  preface 
of  the  previous  edition,  but  have  only  made  such  modi- 
fications and  additions  as  seemed  necessary  to  bring  it 
up  to  the  present  condition  of  science.  Among  these 
additions,  certainly  not  the  least  important  are  the  beau- 
tiful restorations  of  vertebrate  skeletons  and  photo- 
graphs of  natural  objects  for  which  I  am  indebted  to 
Professors  Marsh,  Dean,  Williston,  and  Scott.  I  wish 
hereby  to  thank  these  gentlemen  for  their  hearty  cooper- 
ation. Acknowledgment  is  due  also  to  the  American 
Museum  of  Natural  History,  New  York,  N.  Y.,  for  the 
photograph  of  the  Great  Barrier  Reef,  shown  on  page  97, 

Joseph  Le  Contb, 
Berkeley,  California,  January,  1898. 


CONTENTS, 


PAGE 

Introduction       .        .  7 


Part  I. — Dynamical  Geology. 

CHAPTER 

I.  Atmospheric  Agencies      •        .        .        .                .        ,  10 

IT.  Aqueous  Agencies 17 

III.  Organic  Agencies     . 83 

IV.  Igneous  Agencies 131 


Part  II. — Structural  Geology. 

I.  General  Form  and  Structure  of  the  Earth      ^       .  173 
11.  Stratified  Rocks 479 

III.  Unstratified  or  Igneous  Rocks 210 

IV.  Metamorphic  Rocks 224 

V.  Structures  Common  to  All  Rocks         ....  228 

VI.  Penudation,  or  General  Erosion 252 

Part  III. — Historical  Geology. 

I.  General  Principles 250 

II.  Arch^an  System  and  Archjeozoic  Era         .        .        .  203 

III.  Paleozoic  Rocks  and  Era 267 

IV.  jMesozoic  Era. — Age  of  Reptiles 324 

V.  Cenozoic  Era. — Age  of  Mammals  .        .        .        .        .  363 

VI.   PsYCHOzoic  Era. — Age  of  Man      .....  407 

Index  .       .  417 

5-6 


GEOLOGY. 


INTRODUCTION. 

Definition  of  Geology. — Geology  is  the  science  which 
tieats  of  the  past  conditions  of  the  earth  and  of  its  inhab- 
itants. It  is,  therefore,  a  history  of  the  earth.  It  is 
closely  allied  to  physical  geography,  but  differs  in  this  : 
Physical  geography  treats  only  of  the  present  forms  of  the 
earth's  features ;  geology  also,  and  mainly  of  their  grad- 
ual formation,  or  evolution  fro7n  former  conditions.  It  is 
also  closely  allied  to  natural  history,  but  differs  in  this  : 
Natural  history  is  concerned  only  with  the  present  forms 
and  distribution  of  animals  and  plants,  while  geology  is 
chiefly  concerned  about  previous  forms  and  distribution, 
and  their  changes  to  the  present  forms  and  distribution. 
In  a  word,  geography  and  natural  history  are  concerned 
about  how  things  are  ;  geology,  about  how  they  became  so. 

Cultivates  Habit  of  Observation. — We  have  said 
geology  treats  of  the  history  of  the  past  conditions  of  the 
earth  and  its  inhabitants.  The  evidences  of  the  past  con- 
ditions are  found  in  its  present  structure.  But,  to  under- 
stand this  structure,  we  must  observe  the  manner  in  which 
similar  structure  is  formed  nota  under  our  eyes.  Thus, 
observation  of  causes  noiv  in  operation  constitutes  the 
only  solid  foundation  of  geology.  Fortunately,  the  proc- 
esses by  which  structure  is  now  being  formed  may  be  ob- 
served everywhere ;  and  the  structures  which  have  been 


8  INTRODUCTION. 

thus  formed  in  earlier  times  may  be  observed  in  very  many 
places,  if  we  know  how  to  look  for  them.  Thus  geology, 
perhaps  more  than  any  other  science,  cultivates  the  habit 
of  field-observation  ;  not,  indeed,  that  minute  observation 
required  by  mineralogy  or  botany,  but  that  wider  observa- 
tion which  gives  interest  to  mountain-travel  or  even  to 
rambles  over  the  hills  in  our  vicinity.  It  cultivates  also, 
in  an  eminent  degree,  the  habit  of  tracing  effects  to  their 
causes — for  the  question  ever  present  to  the  geologist  is, 
*'  How  came  it  so  ?  " 

Great  Divisions  of  Geology. — We  have  said  that  the 
history  of  the  earth  is  recorded  in  its  structure,  and  that 
structure  is  understood  by  study  of  causes  or  processes 
now  in  operation.  AVe  have  thus  outlined  the  great  di- 
visions of  geology,  and  the  order  in  which  they  must  be 
studied.  We  must  study,  first  of  all,  causes  and  processes 
now  in  operation  about  us  everywhere,  producing  struc- 
ture. This  is  called  dynamical  geology.  Next,  we  must 
study  the  rocky  structure  of  the  earth  to  as  great  a  depth 
as  we  can,  and  apply  the  previously  acquired  principles  in 
its  interpretation  ;  for  this  structure  has  been  produced 
by  similar  processes  acting  through  all  previous  time. 
This  is  called  structural  geology.  Only  after  this  shall 
we  be  prepared  to  take  up  the  history  of  the  changes 
through  which  the  earth  has  passed  ;  for  this  history  is 
revealed  in  structure.     This  is  called  historical  geology. 


PART  I. 

DYNAMICAL    GEOLOGY. 

As  already  said,  this  part  treats  of  agencies  now  in 
operation  producing  structure.  These  are  best  treated 
under  f©ur  heads — viz.,  atmospheric,  aqueous,  organic, 
and  igneous  agencies.  The  same  agencies  have  operated 
from  the  beginning,  though  probably  with  different  de- 
grees of  activity.  Their  accumulated  effect,  through 
inconceivable  ages,  is  the  present  structure  of  the  earth. 
We  observe  these  operations  now,  in  order  to  understand 
the  effects  of  their  operation  then. 


CHAPTER  I. 

ATMOSPHERIC   AGENCIES. 

Ori^n  of  Soil. — If  we  dig  into  the  earth  anywhere, 
at  a  certain  depth,  greater  in  some  places  than  in  others, 
we  find  rock.  How  was  the  earthy  soil  formed  ?  Per- 
haps some  imagine  that  it  is  an  original  clothing  intended 
to  cover  the  rocky  nakedness  of  the  new-born  earth.  But 
the  very  first  lesson  to  be  learned  by  the  study  of  geology 
is  that  all  things  that  we  see,  even  the  most  enduring — 
such  as  hills,  mountains,  rocks,  etc. — have  hecome  what 
they  are,  usually  by  a  slow  process. 

Now,  soils  are  no  exceptions.  All  soil  is  formed  by  a 
disintegration  or  rotting  doivn  of  rocks.  Sometimes  the 
soils  remain  resting  on  the  rocks  from  which  they  were 
formed  ;  sometimes  they  are  removed  to  another  place,  as, 
e.  g.,  from  hillsides  to  bottom-lands  ;  sometimes  they  are 
carried  by  streams  to  great  distances,  and  deposited  as 
sediments,  and  again  raised  as  land  ;  but  in  all  cases  they 
are  formed  in  the  same  way — viz.,  by  the  rotting  down  of 
rocks  under  the  slow  action  of  the  atmosphere. 

The  active  ingredients  of  the  air  in  this  process  are 
oxygen,  carbonic  acid  (carbon  dioxide),  and  water,  as 
vapor  or  as  moisture.  Now,  rain-water  contains  in  solu- 
tion both  oxygen  and  carbon  dioxide.  Therefore,  rain- 
water, wetting  the  surface  and  penetrating  the  cracks  of 
rocks,  is  the  great  agent  in  the  formation  of  soil. 

Proofs  of  this  Origin  of  Soils. — The  proofs  of  this 
mode  of  formation  are  clearest  in  those  cases  in  which 
10 


ATMOSPHERIC  AGENCIES,  11 

the  soil  still  rests  on  the  rock  from  which  it  was  made. 
Unfortumitely  this  is  rare  in  the  northern  part  of  our 
country,  where  the  soil  has  been  nearly  every  where  s/^i/^e^/ 
during  a  period  which  we  t^hall  hereafter  describe  as  the 
Drift  period.  But  in  the  southern  part  of  the  United 
States,  on  all  the  hillsides  and  mountain-tops,  the  soil 
has  been  undisturbed  for  ages,  and  the  evidence  is  com- 
plete, and  may  be  observed  by  any  one.  If,  for  example, 
we  note  carefully  the  sections  made  by  railroad  and  well 
diggings,  we  shall  see  at  the  io^f  perfect  soil,  perhaps  red  ; 
a  little  deeper  it  becomes  lighter  colored  and  coarser 
grained  ;  then  it  begins  to  look  like  rotten  rock  ;  and, 
finally,  by  insensible  degrees,  it  passes  into  sound  rock. 
The  evidence  is  still  more  complete  if,  as  is  often  the 
case,  the  rock  is  traversed  by  a  quartz-vein.  In  such  a 
case  we  can  trace  the  quartz-vein  through  the  sound 
rock,  and  upward  through  the  rotten  rock,  the  imperfect 
soil,  and  the  perfect  soil,  to  the  surface,  where  it  may 
usually  be  traced  over  hill  and  dale  as  white  fragments 
lying  on  the  surface.  The  reason  is  this  :  Quartz  is  a 
mineral  which  will  not  disintegrate  under  atmospheric 
agency  ;  therefore  it  remains  sound,  while  all  the  rest  of 
the  rock  is  changed  into  soil  (Fig.  1). 


B^lfe; 


Pig.  1.— Section  and  perspective  view  (ideal),  a,  sound  rock  ;  6,  rotten  rock ; 
c,  perfect  soil  ;  d,  quartz-vein  ;  rf',  same,  outcropping  on  surface  ;  e,  mass  of 
more  resistant  rock  imbedded  in  soil. 

Sometimes  a  rounded  mass  of  sound  rock,  e,  is  seen 
imbedded  in   the   soil.     This  is  onjy  a  harder  piece  of 


12  DVNA3IICAL   GEOLOGY. 

rock,  which  has  resisted  disintegration,  while  the  rest  has 
yielded.     These  are  called  bowlders  of  disintegration. 

It  is  not  always,  even  in  lower  latitudes,  that  we  find 
this  gradation  between  soil  and  rock.  Often  perfect  soil 
is  found  to  rest  on  sound  rock,  with  sharp  limit  between. 
In  all  such  cases  there  has  been  shifting  of  the  soil.  In 
northern  latitudes  (37°-40°  northward),  as  already  stated, 
the  soil  nearly  everywhere  rests  on  sound  rock,  and  often 
the  underlying  rock  is  smooth  and  polished.  We  shall 
explain  this  hereafter.  But  even  in  the  Northern  States, 
if  one  will  notice  closely,  he  will  see  the  process  of  soil- 
making  going  on.  Rock-fragments,  which  were  once  an- 
gular, become  rounded  by  rotting  of  the  corners.  Cliffs, 
by  their  crumbling,  gather  piles  of  rock-fragments  and 
earth  (talus)  at  their  bases  (Fig.  2).     The  pupil  ought  to 


Fig.  2.— Cliff,  showing  talus,  t,  and  bowlders  of  disintegration,  b,  b. 

observe  these  things  habitually,  as  it  is  on  just   such 
observation  of  simple  things  that  true  science  rests. 

Depth  of  Soil. — Since  soil  is  constantly  carried  away 
by  washing  of  rain,  as  will  be  more  fully  explained  in  the 
next  chapter,  it  is  evident  that  there  are  two  opposite 
processes  here  to  be  considered,  viz.,  soil-formation  and 
soil-removal.  The  depth  of  the  soil  will  depend  on  the 
relation  of  these  two  to  each  other.  More  definitely,  the 
depth  of  the  soil  depends  partly  upon  the  kind  of  rock 
(for  this  affects  the  rate  of  formation),  and  partly  on  the 
slope  (for  this   affects   the  rate  of  removal).     On  high 


ATMOSPHERIC  AGENCIES. 


13 


slopes  the  rock  is  bare  (Fig.  3,  a),  not  because  there  is  no 
soil  formed,  but  because  it  is  removed  as  fast  as  formed. 


V  \ 


'■■>C< 


ii^Xh^rfa^^r. 


Pio.  3. — rt,  sound  rock  ;  b,  rotten  rock ;   c,  soil  formed  in  place  ;  d,  soil 
shifted  from  e. 


On  flat  lands,  near,  high  slopes,  the  soil  is  deep  (Fig.  3,  5), 
because  not  only  is  it  formed  here  in  place,  but  the  wasn- 
ings  from  above  are  added. 

Kate  of  Disinteg-ration. — If  rocks  were  solid,  so  that 
the  agents  of  decomposition  could  act  only  on  the  sur- 
face, the  rate  would  be  inconceivably  slow,  but  all  rocks 
are  affected  with  joints  in  several  directions,  by  which 
the  mass  is  divided  into  more  or  less  separable  blocks,  so 
that  a  cliff  looks  something  like  a  wall  of  regularly  piled 
blocks  without  cement  (Fig.  2).  Water,  therefore,  pene- 
trates to  great  depths,  attacking  the  surface  of  every 
block.  Also,  every  block  is  itself  affected  throughout 
with  capillary  fissures,  through  which  water  penetrates  to 
every  part  (quarry-water  of  stone-cutters).  Thus,  the 
rocky  crust  of  the  earth  is  affected  by  disintegrating 
agencies  to  very  great  depths — though,  of  course,  most 
rapidly  at  the  surface. 

Bowlders  of  Disiiitegration. — All  over  the  Northern 
States  are  found  scattered  rock-masses  (bowlders),  lying 
on  the  surface.  If  we  examine  these,  we  shall  usually 
find  that  they  are  entirely  different  from  the  country- 
rock.  They  have  been  brought  from  a  distance — how, 
wti  s]i3,ll  explain  hereafter.     We  have  nothing  to  do  with 


14  DYNAMICAL  GEOLOGY. 

these  now.  But  in  the  Southern  States  also,  in  many- 
places,  are  found  huge,  isolated  masses,  lying  on  the  sur- 
face, and  even  sometimes  forming  rocking  stones  (Fig.  4). 
If  we  examine  these,  we  find  that  they  are  of  the  same 


Pig.  4. 

material  as  the  country-rock.  They  have  been  formed  171 
place.  In  the  general  disintegration  of  rock,  and  forma- 
tion and  removal  of  soil,  these  have  resisted,  because 
harder  than  the  rest.  Nothing  is  more  interesting  than 
thus  to  trace  the  configuration  of  the  surface  of  the 
country  to  unequal  resistance  to  atmospheric  agencies. 

Explanation  of  Rock-Disintegration. — If  we  take  a 
piece  of  old  and  very  hard  mortar,  and  pour  on  it  a  little 
hydrochloric  acid,  it  quickly  breaks  down  into  sand,  wet 
with  a  solution  of  calcium  chloride.  The  explanation  is 
simple.  Mortar  consists  of  grains  of  sand  cemented  into 
a  mass  by  hydrate  or  carbonate  of  lime.  The  acid  dis- 
solves the  lime-cement,  and  the  mass  falls  to  powder. 
Now,  mortar  is  really  artificial  stone,  and  nearly  all  rock  is 
constituted  in  a  similar  manner,  i.  e.,  consists  of  particles 
cemented  together.  In  all  rock  some  parts  are  soluble  in 
atmospheric  water,  and  some  are  not.  Under  the  long- 
continued  action  of  this  agent,  therefore,  the  soluble 
parts  are  dissolved,  and  the  mass  breaks  down  into  a 
powder,  or  dust  of  the  insoluble  parts,  wet  with  a  solu- 
tion of  the  soluble  parts.  The  main  difference  between 
the  experimental  and  the  natural  case  is,  that  in  one  the 
process  is  rapid,  and  in  the  other  extremely  slow. 

Examples. — One  or  two  examples  will  make  this  plain  : 


ATMOSPHERIC  AGENCIES.  15 

1.  Sandstone  is  a  rock  made  up  of  grains  of  sand  cemented 
into  a  mass,  sometimes  by  lime  carbonate,  sometimes  by- 
silica.  Under  the  slow  action  of  atmospheric  water  the 
cement  is  dissolved,  and  the  rock  crumbles  into  sand, 
moistened  with  a  solution  of  lime  carbonate,  if  this  be  the 
cement.  3.  Granite  and  gneiss  and  many  other  igneous 
and  metamorphic  rocks,  such  as  are  found  on  the  eastern 
slope  of  the  Appalachian  Chain  everywhere,  are  an  aggre- 
gation of  four  minerals,  viz.,  quartz,  feldspar,  mica, 
and  hornblende.  In  coarse  granite  these  can  be  easily 
seen  with  the  naked  eye.  The  bluish  glassy  specks  are 
quartz ;  the  opaque  white,  or  rose-color,  are  feldspar  ; 
the  glistening  scales  are  mica  ;  and  the  black  spots,  horn- 
blende.* The  whole  rock  may  be  regarded  as  grains  of 
quartz,  mica,  and  hornblende  cemented  into  a  mass  by 
feldspar.  IS  ow,  quartz  is  not  at  all,  and  mica  very  slightly, 
affected  by  atmospheric  water  ;  but  the  feldspar  and 
hornblende  are  slowly  changed  into  clay,  which,  in  the 
case  of  hornblende,  is  red,  from  the  presence  of  iron. 
Thus,  the  whole  rock  rots  down  to  a  clay  soil,  usually 
red,  in  which  are  disseminated  grains  of  quartz  and 
scales  of  mica,  the  whole  moistened  with  water,  contain- 
ing in  solution  a  little  potash  derived  from  the  feldspar. 
This  is  the  commonest  of  all  soils.  3.  Slates  and  shales 
are  clays  hardened  into  rock  by  some  cement  such  as 
lime  or  silica.  When  the  cement  is  dissolved  the  rock 
crumbles  into  a  clay  soil.  4.  A  pure  limestone  like  mar- 
ble makes  no  soil  because  it  is  all  soluble,  but  most  lime- 
stones are  mixed  with  clay  or  sand.  When  the  lime  is 
dissolved  the  result  is  a  limy  clay  or  limy  sand. 

Mechanical  Action  of  Air;  Frosts. — The  soil-for- 
mation, above  explained,  is  a  chemical  process,  but,  in 
cold  climates  and  mountain-regions,  atmospheric  water 
acts  also  mechanically  and  very  powerfully  in  rock-dis- 

*  These  minerals  ought  to  be  shown  the  pupil,  both  separately  and 
as  aggregated  in  a  specimen  of  coarse  granite. 


16  DYNAMICAL   GEOLOGY. 

integration.  Water  penetrating  the  joints,  and  freezing, 
expands  with  such*force  that  the  rocks  are  riven  asunder  ; 
and  then,  penetrating  again  into  the  capillary  fissures 
and  freezing,  these  blocks  are  in  their  turn  broken  into 
smaller  fragments,  until  the  whole  crumbles  to  dust. 

Wind. — Again,  loose  earth,  sand,  and  dust,  especially  in 
dry  climates,  are  carried  by  winds,  and  sometimes  accu- 
mulate in  large  quantity  and  form  a  peculiar  soil.  Thus, 
the  sands  of  Sahara  are  in  some  places  encroaching  on 
the  fertile  lands  of  Egypt.  Thus,  also,  sea-sands  are 
often  carried  inland  from  shore,  and  cover  up  and  destroy 
fertile  lands.  The  sand-hills  to  the  west  of  San  Fran- 
cisco are  made  in  this  way.  The  phenomena  of  sand- 
dunes  may  be  observed  in  many  places  along  the  coasts 
of  nearly  all  countries.  Some  geologists  think  that  in  the 
interior  of  dry  countries,  like  Asia  or  the  western  part  of 
our  own  country,  soil  of  great  thickness  has  been  formed 
by  accumulation  oi  dust. 


CHAPTER  II. 

AQUEOUS  AGENCIES. 

Aqueous  and  atmospheric  agencies  are  so  closely  con- 
nected that  many  treat  them  together  under  the  one  head 
^A  leveling  agencies.  Water,  as  atmospheric  moisture  or 
as  rain,  soaking  into  the  earth,  is  the  chief  agent  of  soil- 
making  ;  but  water,  falling  more  abundantly,  runs  off  the 
surface,  and  is  also  the  chief  agent  of  soil-removaL  In 
the  one  case  it  acts  as  a  chemical,  in  the  other  as  a  me- 
chanical, agent.  The  agency  of  water  in  soil-making  we 
treated  under  atmosjjlteric,  its  agency  in  soil-removal  be- 
longs to  aqueous,  agencies.  The  one,  acting  at  all  times 
and  in  all  places,  its  effects  are  obscure  and  inconspicu- 
ous ;  the  other,  acting  occasionally  and  concentrating  its 
power  on  particular  places,  its  effects  are  easily  observed 
and  better  understood.  Nevertheless,  the  aggregate  ef- 
fects of  the  one  must  be  equal  to  those  of  the  other,  for 
the  former  prepares  the  way  for  the  latter.  Aqueous 
agencies  have  little  effect  upon  rocks  unless  they  have 
been  first  rotted  down  to  soils. 

Although  the  agency  of  water  is  mainly  mechanical, 
yet  there  is  a  chemical  agency  of  water  other  than  that 
of  soil-making.  The  agency  of  water  may  therefore  be 
divided  into  mechanical  and  chemical.  The  mechanical 
agency  is  best  treated  under  the  three  heads  of  rivers, 
ocean,  and  ice,  and  each  of  these  again  in  cutting  away, 
in  carrying,  and  in  throwing  down  again,  or  in  erosion, 
tra7isportation,  and  deposit.  The  chemical  agency  we 
shall  consider  under  the  two  heads  of  chemical  deposits 
in  sprmgs  and  in  lakes : 

Le  Contb,  Geol.  2 


18  DYNAMICAL  GEOLOGY. 

i  Rivers,  erosion,  transportation,  deposit. 
Ocean,        "  "  " 

Ice, 

^'^*^^'^^'   k  Qiieraical f  ^P^^igs,  chemical  deposits  in, 

^  \  Lakes,  '«  " 

Section  I. — Rivers. 

Atmospheric  or  meteoric  water  falls  on  land  as  rain.  A 
portion  sinks  into  the  earth,  and,  after  a  longer  or  shorter 
subterranean  course  and  doing  its  appropriate  work  of 
rock-disintegration  and  soil-making,  comes  up  again  to 
the  surface  as  springs.  Another  portion  runs  off  the 
surface,  cutting  and  carrying  away  the  soil  everywhere. 
Quickly,  however,  it  gathers  into  rills  and  cuts  furrows, 
these  rills  uniting  into  streamlets  and  cutting  gullies. 
The  streamlets,  uniting  with  each  other,  and  with  water 
issuing  from  springs,  form  mountain-torrents,  and  cut  out 
great  ravines,  gorges,  and  caflons.  Finally,  the  torrents, 
emerging  on  the  plains  from  their  mountain  home,  form 
great  rivers,  which  deposit  their  freight  of  gathered  earth 
and  rock-fragments  in  their  courses,  and  finally  in  the  sea 
or  lake  into  which  they  empty.  Such  is  a  condensed  his- 
tory of  the  course  and  work  of  water  from  the  time  it  falls 
as  rain  until  it  reaches  the  ocean  from  which  it  came.  All 
of  this  we  include  under  river-agency.  It  may  be  defined 
as  the  work  of  rain  and  rivers,  or  the  work  of  circulating 
meteoric  water.  All  that  follows  on  this  subject  will  be 
but  an  expansion  of  the  condensed  statement  given  above, 
and  much  of  it  may  be  observed  by  any  one  who  does 
not  commit  the  mistake  of  thinking  things  insignificant 
because  they  are  common. 

1.  Erosion  of  Rain  and  Rivers. 

The  rain  which  falls  on  land-surface  may  be  divided 
into  three  parts  :  One  part  runs  immediately  from  the 
surface,  producing  universal  rai^i-erosion  and  the  muddy 


AQUEOUS  AGENCIES,  19 

floods  of  the  rivers.  Another  part  sinks  into  the  earth, 
and,  after  doing  its  appointed  work  of  soil-making,  re- 
appears on  the  surface  as  springs,  and  forms  the  ordinary 
flow  of  rivers  in  dry  times.  This  part  joins  the  surface 
drainage,  and  together  they  concentrate  their  work  along 
certain  lines,  and  thus  produce  stream-erosio7i.  A  third 
portion  never  reappears  on  the  surface,  but  finds  its  way, 
by  subterranean  passages,  to  the  sea. 

By  the  continued  action  of  rain  and  rivers  all  lands 
(except  some  rainless  deserts)  are  being  cut  away  and 
carried  to  the  sea.  Every  one,  each  in  his  own  vicinity, 
may  see  this  process  going  on.  The  soil  of  the  hillsides 
is  everywhere  being  washed  away  by  rain,  and  carried  off 
in  the  muddy  streams.  At  what  average  rate  is  this  wash- 
ing process  going  on  ?  This  is  a  question  of  extreme 
importance. 

Average  Rate  of  Erosion. — By  observations  made  on 
rivers  in  all  parts  of  the  world  it  has  been  estimated  that 
all  land-surfaces  are  being  cut  away  at  a  rate  of  about 
one  foot  in  3,000  to  5,000  years.  The  Mississippi  cuts 
down  its  whole  drainage-basin  one  foot  in  5,000  years,  the 
Ganges  one  foot  in  2,000  years.  Some  rivers  cut  still 
more  rapidly,  but  most  less  rapidly  than  these.  The  rate 
differs  in  different  parts  of  the  same  basin.  In  mountain- 
regions  the  rate  is  at  least  three  times  the  average  given 
above,  and  on  steeper  slopes  still  greater.  On  the  lower 
plains  the  erosion  is  small,  and  in  many  places  there  is 
deposit  instead  of  erosion.  Making  due  allowance  for  all 
these  variations,  it  is  probable  that  all  land-surfaces  are 
being  cut  down  and  lowered  by  rain  and  river  erosion  at 
a  rate  of  one  foot  in  5,000  years.  At  this  rate,  if  we  take 
the  mean  height  of  lands  as  1,200  feet,  and  there  be  no 
antagonistic  agency  at  work  raising  the  land,  all  lands 
would  be  cut  down  to  the  sea-level  and  disappear  in 
6,000,000  years. 

This  universal  cutting  away  of  land-surfaces  we  have 


20  DYNAMICAL  GEOLOGY, 

divided  for  convenience  into  two  parts,  which,  however, 
graduate  completely  into  each  other — viz.,  rain-erosi07i 
and  dream-erosion :  the  one  is  universal,  but  small  and  in- 
conspicuous in  any  one  place;  the  other  is  confined  to 
water-channels,  but  works  with  concentrated  and  con- 
spicuous effects.  The  one  may  be  compared  to  a  univer- 
sal sand-paperi7ig,  the  other  to  the  action  of  the  graver's 
tool,  cutting  ever  deeper  along  the  same  lines.  Of  the 
two,  the  general  rain-erosion,  though  less  conspicuous,  is 
probably  far  the  greater  in  aggregate  amount.  They  co- 
operate in  cutting  away  the  land,  and,  if  unopposed,  would 
finally  destroy  it.  Pure  loater,  however,  has  comparatively 
little  effect.  Its  graving-tools  are  the  sand,  gravel,  peb- 
bles, and  rock-fragments,  which  it  carries  along  in  its 
course. 

Conspicuous  Examples  of  Stream-Erosion. --The 
effects  of  erosion  are  most  conspicuously  seen  in  water- 
falls, ravines,  gorges,  and  caflons  ;  but  also,  in  less  degree, 
on  every  hillside,  and  in  every  furrow  and  gully. 

Waterfalls  ;  Niag-ara. — The  Niagara  Falls  and  gorge 
are  an  instructive  example  of  stream-erosion,  because  the 
effects  are  easily  observed  from  year  to  year. 

General  Configuration  of  the  Country. — Lake  Erie 
is  situated  on  a  nearly  level  plateau,  several  hundred  feet 
above  a  similar  plateau,  on  which  is  situated  Lake  On- 
tario. The  plateaus  are  separated  by  an  almost  perpen- 
dicular cliff,  running  east  and  west,  near  Lake  Ontario. 
The  Niagara  River  runs  out  of  Lake  Erie,  and  on  the 
Erie  plateau,  fifteen  to  eighteen  miles,  then  drops,  by  a 
perpendicular  fall,  into  a  narrow  gorge,  with  nearly  per- 
pendicular sides,  and  runs  in  the  gorge  for  seven  miles, 
and  then  emerges  on  the  Ontario  plateau  just  before 
emptying  into  that  lake.  Fig.  5  is  an  ideal  section  through 
the  middle  of  the  river,  and  showing  these  facts.  The 
light  lines  show  the  cliffs  on  the  other  side  of  the  gorge. 

Recession  of  the  Falls. — Ever  since  their  discovery, 


AQUEOUS  AGENCIES.  21 

200  years  ago,  the  falls  have  steadily  worked  their  way 
back  toward  Lake  Erie.  The  rate  of  recession  has  been 
estimated  at  one  to  three  feet  per  annum.  The  cause 
is  easily  perceived.     The  strata  at  the   falls  consist  of 


Fig.  5.— Section  of  Niagara  Falls  and  gorge.     O.P.,  Ontario  plateau  ;  E.P.,  Erie 
plateau  ;  L.O,,  Lake  Ontario  ;  /,  fall ;  mb,  stratified  mud-banks. 


solid  limestone,  represented  in  the  figure  by  the  jointed 
structure,  underlaid  by  softer  shale.  The  force  of  the 
dashing  water  cuts  away  the  soft  shale  and  undermines 
the  limestone,  causing  it  to  project  as  overhanging  rocks, 
which  fall  from  time  to  time  into  the  abyss  below.  Thus 
the  falls  work  backward,  but  remain  perpendicular. 

Gorge  formed  by  Recession. — There  can  be  no  doubt 
that  the  whole  gorge  has  been  formed  in  this  way  ;  that 
the  river  once  fell  over  the  cliff  which  runs  across  its 
course  near  Lake  Ontario,  and  then  worked  its  way  back 
to  its  present  position ;  and  the  work  is  still  going  on. 
The  general  configuration  of  the  country  suggests  this 
origin  even  to  the  casual  observer,  and  close  examination 
entirely  confirjns  it.  It  is  a  familiar  fact  that  stratified 
mud-banks  are  found  in  spots  along  the  margins  of  all 
rivers,  evidently  formed  by  deposits  from  the  river.  These 
stratified  muds  often  contain  the  shells  of  the  mussels 
which  inhabit  the  river.  Now,  in  several  spots  {mb) 
along  the  top  of  the  gorge-cliff,  from  the  falls  to  Lake 
Ontario,  are  found  such  stratified  mud-deposits  contain- 
ing shells.  The  deposits  were  evidently  made  when  the 
river  ran  at  that  level. 


^^  DYNAMICAL  OEOLOa^. 

Time. — Several  attempts  have  been  made  to  estimate 
the  time  occupied  in  this  process.  Mr.  Lyell  estimated  it 
at  35,000  years.*  A  large  part,  if  not  the  wnole  of  this 
time,  belongs  to  the  present  geological  epoch,  and  was 
probably  witnessed  by  early  man. 

Other  Falls. — Many  other  perpendicular  falls  have 
receded  in  a  similar  way  and  given  rise  to  similar  gorges. 
The  most  remarkable  of  these  are  the  Falls  of  St.  Anthony, 
The  Mississippi  River,  at  Fort  Snelling  (mouth  of  the 
Minnesota  River),  is  traversed  by  an  escarpment  which 
separates  a  higher  from  a  lower  plateau.  The  river  runs 
on  the  upper  plateau  as  far  as  Minneapolis,  then  drops, 
by  a  nearly  perpendicular  fall,  into  a  goige  one  hundred 
feet  deep,  runs  in  this  gorge  eight  miles,  and  then  emerges 
on  the  lower  plateau  at  Fort  Snelling.  Here,  again,  we 
have  the.  upper  plateau  capped  by  a  hard  limestone,  under- 
laid by  a  soft  sandstone.  Here,  also,  the  wearing  away  of 
the  underlying  sandstone  causes  the  limestone  to  project 
in  overhanging  tables  which  fall  from  time  to  time  into 
the  chasm  below,  and  so  the  fall  works  backward.  There 
is  no  doubt  that  the  Mississippi  at  one  time  fell  over  the 
escarpment  at  Fort  Snelling,  and  has  worked  its  way  back 
to  its  present  position,  and  that  this  all  took  place  during 
the  present  geological  epoch,  and  while  man  inhabited  the 
continent.  Professor  Winchell  has  estimated  that,  at  its 
present-rate  recession,  it  would  take  not  more  than  8,000 
years  to  accomplish  the  work. 

Minnehaha  River  is  a  tributary  running  into  the  Mis- 
sissippi about  six  miles  below  the  falls.  It  therefore,  at 
one  time,  fell  into  the  gorge.  It  has  now  worked  itself 
back  about  two  miles,  and  forms  the  beautiful  ^^  Minne- 
haha Falls,''  made  celebrated  by  their  description  in  Long- 
fellow's ^'Hiawatha." 

The  Columbia  River,  where  it  breaks  through  the 
Cascade  Range,  has  cut  a  gorge  fifty  miles  long  iind  1,000 
*  Later  estimates  make  it  about  11,000  years. 


AQUEOUS  AGENCIES.  23 

to  3,000  feet  deep.  All  the  tributaries  which  run  into  the 
river  at  this  point  have  cut  deep  side  gorges,  headed  by 
perpendicular  falls.  Some  of  the  most  exquisite  falls  are 
here  nestled  among  the  hills  in  these  almost  inaccessible 
gorges.  The  country  rock  is  a  very  hard  but  much 
jointed  lava,  underlaid  by  a  softer  cement-gravel.  The 
falls  have  eaten  out  the  gravel  and  undermined  the  lava, 
which  from  time  to  time  tumbles  into  the  chasm  as  blocks 
that  are  carried  away  by  the  stream.  In  this  way  the 
falls  have  worked  back  about  two  miles. 

Yoseinite  Falls. — Most  perpendicular  falls  have  been 
made  by  recession,  as  explained  above,  but  this  is  not  true 
of  all.  The  Yosemite  Falls  (of  which  there  are  six,  vary- 
ing in  height  from  400  to  1,600  feet)  have  not  perceptibly 
receded.  This  is  because  the  granite  is  very  hard,  and 
the  time  too  short  (probably  only  a  few  thousand  years), 
since  the  valley  was  filled  with  ice  (page  394). 

Ravines,  Gorges,  Canons. — These  are  found  in  all 
countries,  especially  in  mountainous  and  high-plateau  re- 
gions. They  are  always  or  nearly  always  formed  by  run- 
ning water,  although  in  some  cases  their  places  are  deter- 
mined by  fractures  of  the  earth's  crust  (page  229).  They 
are  gullies  on  a  large  scale.  In  the  Appalachian  Chain 
the  most  striking  examples  are  the  Hudson  River  goige  in 
New  York,  the  Tallulah  gorge  in  Georgia,  and  the  French 
Broad  gorge  in  North  Carolina.  But  it  is  in  the  western 
part  of  the  continent  that  the  finest  examples  are  seen. 
Nowhere  in  the  world  are  they  on  a  grander  scale,  more 
evidently  due  to  water  alone,  or  more  recent  in  origin. 
As  we  are  studying  ^^  causes  now  in  operation,^'  they  are 
the  most  instructive  examples  to  be  found  anywhere. 

In  California  there  was,  even  since  middle  geological 
times,  an  old  river-system  different  from  the  present. 
This  will  be  explained  more  fully  hereafter  (page  395V 
These  old  river-valleys  were  filled  up  with  river-gravel, 
and  finally  obliterated  by  lava-flows  not  long  before  tiie 


24  DYNAMICAL   GEOLOGY. 

advent  of  man.  The  displaced  rivers  have  since  that  time 
cut  new  channels,  far  deeper  than  the  old,  so  that  the  old 
lava-covered  channels  are  high  up  on  the  present  divides 
(Fig.  6).     Thus,  in  very  recent  geological  times — i.  e.,  in 


Fig.  6.— Section  across  old  and  new  river  beds  of  California,  r,  r,  new  river  beds ; 
r\  old  river  bed;  gr,  gravels  of  present  rivers;  gr't  old  river  gravels;  dotted  line, 
old  configuration  of  surface. 


the  Quaternary  and  present  epochs — water  has  cut  at  least 
2,000  feet  deep  in  hard  slate-rock. 

We  have  selected  these  cases  because  of  the  plain  evi- 
dence of  recent  work,  but  the  whole  western  slope  of  the 
Sierra  is  trenched  with  enormous  ravines,  3,000  to  6,000 
feet  deep,  although  the  history  of  some  of  them  is  longer 
than  those  spoken  of  above.  For  example,  commencing 
north  and  going  southward,  we  have  the  Columbia  River, 
with  its  gorge  3,000  feet  deep  in  hard  lava.  The  branches 
of  the  Feather,  Yuba,  and  American  Rivers  have  cut 
gorges  2,000  to  3,000  feet  deep  in  hard  slate.  These  have 
the  structure  represented  by  Fig.  6,  and  have  been  cut 
wholly  in  very  recent  geological  times.  The  Tuolumne 
and  Merced  Rivers  have  cut  gorges  3,000  to  5,000  feet 
deep,  the  famous  Hetch-hetchy  and  Yosemite  Valleys 
being  in  the  course  of  these.  King^s  River  Canon  is 
7,000  feet  deep,  in  hard  granite. 

Plateau  Region. — But  the  most  wonderful  gorges  or 
cations  in  the  world  are  found  in  the  high-plateau  region — 
i.  e.,  the  region  between  the  Colorado  and  Wahsatch 
Mountains,  and  drained  by  the  Colorado  River.  This 
region  is  6^000  to  8^000  feet  high,  and  consists  of  nearly 


AQUEOUS  AGENCIES! 


25 


level  strata,  which  have  been  cut  into  by  the  Colorado  and 
its  tributaries  in  such  wise  that  the  whole  river  system  of 
the  country  runs  far  below  the  general  level.  The  Grand 
*  Caflon  of  the  Colorado  is  300  miles  long  and  3,000  ^o 
6,000  feet  deep,  and  all  its  tributaries  come  in  by  side 
cafions  of  almost  equal  depth  (Fig.  7). 


Fig.  7.— View  of  Colorado  Canon  and  its  tributaries,  with  erosion-columns  and 
mesas  in  the  distance. 


Besides  this  prodigious  stream  cutting,  the  general  rain- 
erosion  has  been  here  upon  an  equally  grand  scale.  Many 
thousands  of  feet  have  been  (tarried  away  over  the  whole 
area  of  about  1 00,000  square  miles  or  more.  This  is  shown 
by  the  isolated  peaks  and  tables  of  level  strata  scattered 
about,  and  still  better  by  the  succession  of  cliffs  shown  in 


26  DYNAMICAL   GEOLOGY. 

Fig.  156  (page  250),  as  will  be  more  fully  explained  here- 
after. 

Time. — The  time  during  which  the  whole  of  this  enor- 
mous work  was  done  is  but  a  small  portion  of  the  geolog- 
ical history.  It  commenced  in  Middle  Tertiary  (page 
383),  continued  to  the  present  time,  and  is  still  going  on. 

Pot  holes. — If  we  examine  the  bare  rocky  beds  of 
swift  streams  in  mountain  regions,  we  often  find  deep 
holes  with  vertical  walls  like  small  rock  wells.  In  their 
bottoms  we  are  sure  to  find  gravel  and  a  good  many 
rounded  pebbles.  These  are  called  pot  holes.  They  are 
formed  thus  :  Swift  streams  form  whirling  eddies,  in 
which  sand,  gravel,  and  rock  fragments  carried  by  the 
stream  are  whirled  about  in  the  same  spot  until  they  hol- 
low out  these  holes,  while  the  fragments  themselves  are 
rounded  into  pebbles  in  the  process.  These  become  signs 
of  old  river  beds  where  rivers  no  longer  exist. 

2.   Transportation  and  Distribution  of  Sediments, 

Kiver  agency,  it  will  be  remembered,  is  taken  up  under 
three  heads.  We  have  already  taken  up  one — Erosion. 
The  other  two  are  best  taken  up  together,  as  Transporta- 
tion and  Distribution  of  Sediments. 

Transporting  Power  of  Water. — Every  one  is  fa- 
miliar with  the  fact  that  running  water  carries  along 
materials  of  different  degrees  of  fineness,  but  the  rate  at 
which  the  carrying  or  lifting  power  increases  with  the 
velocity  is  almost  incredible  to  those  who  have  not  inves- 
tigated the  subject.  It  is  found  that  the  size  or  weight 
of  the  separate  particles  or  fragments  movable  by  running 
water  increases  at  the  enormous  rate  of  the  sixth  power  of 
the  velocity  of  the  current.  Thus,  if  the  velocity  of  a 
current  be  doubled,  it  can  carry  a  stone  sixty-four  times 
as  great  as  before  ;  if  it  be  increased  ten  times,  it  can 
carry  a  stone  1,000,000  times  as  great  as  before.     We  can 


AQUEOUS  AGENCIES.  27 

thus  easily  understand  the  prodigious  power  of  mountain- 
torrents  when  swollen  by  heavy  rains. 

It  follows  from  the  above  that,  if  a  stream  be  carrying 
all  it  can,  the  least  checking  of  its  velocity  will  cause 
abundant  deposit,  and  the  least  increase  of  its  velocity 
will  cause  it  to  take  up  again  what  it  had  previously 
deposited — i.  e.,  it  will  scour  its  bed  and  banks. 

Sorting-  Power  of  Water. — If  we  take  a  handful  of 
earth  and  throw  it  into  a  deep  basin,  and,  after  allowing 
it  to  settle,  pour  off  the  water  and  examine  the  sediment, 
we  shall  find  that  it  is  neatly  sorted,  the  coarser  particles 
being  at  the  bottom,  and  above  this  finer  and  finer,  until 
a  very  fine,  smooth  mud  forms  the  top.  The  earth  will 
be  still  better  sorted  if  we  throw  it  into  running  water. 
In  this  case  the  coarser  will  drop  first,  i.  e.,  higher  up, 
and  the  finer  lower  and  lower,  until  only  the  finest  will  be 
carried  far  down  the  stream.  This  is  especially  the  case 
if  the  velocity  decreases  as  we  go  down-stream,  as  is 
usually  the  case  in  natural  streams.  Thus,  pebbles  are 
found  in  torrent-beds,  and  fine  mud  in  lower  parts  of 
streams. 

Stratification. — If  we  examine  carefully  the  mud  or 
sand  of  a  river-bank  or  lake-margin,  we  shall  always  find 
them  stratified,  i.  e.,  in  layers  of  slightly  different  color 
and  grain.  This  is  easily  explained  by  the  sorting  power 
of  water.  If  the  water  be  still,  as  in  a  lake  or  pond,  then 
with  every  rain  earth  is  brought  in,  and  by  settling  is 
sorted,  the  finest  falling  last.  Thus  the  coarse  material 
of  one  rain  falls  on  the  fine  of  the  previous  rain,  and 
every  rain  is  marked  by  a  separate  layer.  In  rivers,  the 
same  r3sult  follows,  but  the  explanation  is  a  little  differ- 
ent. The  velocity  of  the  current  is  changing  from  day 
to  day  on  account  of  the  varying  supply  of  water.  The 
stream-lines  also  are  continually  shifting  from  side  to 
side.  Thus  the  velocity  at  any  one  point  is  all  the  time 
changing,   and  therefore  the  character  of   the   material 


38  DYNAMICAL  GEOLOOY, 

deposited  is  also  changing  from  day  to  day^  and  even 
from  hour  to  hour — now  coarser,  now  finer — and  a  very 
distinct,  though  often  irregular,  stratification  is  the  result. 
General  Law. — We  may  therefore  state  it  as  a  general 
law  that  all  deposits  in  tuater,  whether  still  water,  as 
lakes  and  seas,  or  running  water,  as  rivers,  are  stratified, 
and,  conversely,  that  all  stratified  7naterials,  wherever  we 
find  them,  whether  near  water  or  high  up  on  the  tops  of 
mountains,  and  in  whatsoever  condition  we  find  them, 
whether  as  sands  and  muds  or  as  hard  stone,  if  the  strati- 
fication be  a  true  stratification,  i.  e.,  the  result  of  sorted 
material,  have  been  deposited  in  wafer.  Upon  this  very 
simple  law  nearly  the  whole  of  geological  reasoning  is 
based.  It  is  important,  therefore,  that  every  one  should 
habitually  observe  the  phenomena  described  above,  not 
only  in  lakes  and  rivers  but  in  shower-rills  and  pools. 
VYe  are  now  in  position  to  explain  all  the  phenomena  of 
rivers. 

1.  Final  Effect  of  Erosion  of  Rain  and  Rivers. 

There  is  a  certain  slope  of  a  river-bed,  depending  on 
the  amount  of  sediment  carried,  at  which  the  river  neither 
cuts  nor  deposits.  This  is  called  its  base-level  of  erosion. 
Every  river  is  seeking  this  level.  If  above  it,  it  seeks  to 
reach  it  by  cutting ;  if  below  it,  by  building  up  by  sedi- 
mentation. As  soon  as  the  river  reaches  this  level,  it 
rests,  so  far  as  down-cutting  is  concerned,  but  now  begins 
to  sweep  from  side  to  side,  widening  its  channel.  Mean- 
while the  side  streams  and  rain-wash  continue  to  cut 
down  the  divides.  If  this  continues  without  interrupt 
tion,  the  final  result  is  a  gently  undulating  land-surface 
with  low  divides  and  broad  river  channels.  This  is  called 
a  Peneplain.  It  means  that  the  land  has  remained 
•steady  and  the  rivers  have  been  working  on  it,  a  long 
time.  It  is  old  topography.  If  the  land  be  now  elevated 
by  interior  forces  the  rivers  increase  in  velocity  and  begin 


AQUEOUS  AGENCIES.  39 

cutting  again  and  form  deep  caflons.  These  deep  cafions 
are  therefore  characteristic  of  new  topography — i.  e.,  of  a 
rising  or  newly  risen  land  and  of  rivers  far  above  their 
base-level  and  working  hard  to  reach  it.  If  on  the  other 
hand  the  land  sinks,  the  rivers  become  more  sluggish^ — 
they  cease  to  cut  and  begin  to  build  up  by  deposit.  Thus 
river  channels  become  delicate  indicators  of  the  move- 
ments of  the  earth's  crust. 

2.    Windiiig  Course  of  Rivers. 

The  winding  course  of  rivers  is  the  necessary  result  of 
the  laws  of  currents.  Streams  do  not  find  irregular  chan- 
nels to  which  they  are  forced  to  conform,  but  they  make 
their  own  channels.  If  we  straighten  these  channels, 
they  will  not  remain  straight.  Some  point  will  wear  into 
a  hollow.  This  will  throw  the  stream  to  the  other  side, 
which  will  in  like  manner  be  worn,  and  thus  the  stream 
begins  to  meander.  Now,  if  we  examine  any  winding 
stream,  we  shall  see  that  the  swiftest  current  is  on  the 
outer  part  of  the  curve  and  the  slowest  on  the  inner  side, 
or,  in  other  words,  the  current  is  swifter  than  the  aver- 
age on  the  outer  and  slower  than  the  average  on  the  inner 
side  of  the  bends.     In  the  figure,  the  arrows  show  the 


Fig.  8. 


line  of  swiftest  current.  Noav,  if  the  river  is  carrying 
all  the  sediment  its  average  velocity  can,  it  is  evident  that 
it  will  cut  on  its  outer  curve,  where  the  velocity  is  greater 
than  the  average,  and  deposit  and  make  land  on  the 
inner  side,  where  the  velocity  is  less  than  the  average. 
Thus  the   outer  curve  is  increased  by  erosion  and  the 


80 


DYNAMICAL  GEOLOGY. 


inner  curve  by  deposit,  and  the  winding  tends  ever  to 
become  greater  and  greater.  This  is  most  conspicuous 
in  cases  in  which  rivers  run  between  mud-banks  made  by 
their  own  deposit.  In  such  cases,  the  curves  become 
greater  and  greater,  until  finally  two  contiguous  curves 
cut  into  each  other,  the  river  straightens  itself,  and  the 
old  bend  is  thrown  out  and  becomes  a  lagoon  (d,  Fig  9). 


Pig.  9 — a,  b,  c,  successive  stages  in  the  winding  course  of  a  river. 

Many  such  lagoons  exist  in  all  rivers  which  run  through 
swamp-lands.  Fig.  9  shows  the  process,  and  Fig.  10  is  a 
portion  of  the  lower  Mississippi  Eiver  showing  the  result. 


Fig.  10.— a  portion  of  lower.  Mississippi, 

3.  Flood-Plains  and  Their  Deposits. 

Rivers  usually  rise  in  hilly  or  mountainous  regions,  and 
flow  in  the  lower  course  through  flat  plains.  In  flood 
seasons,  the  velocity  being  checked  by  change  of  slope, 
the  channels  are  no  longer  able  to  contain  their  waters, 
which  therefore  overflow  portions  of  the  flat  lands  on  each 
side.  The  area  liable  to  overflow  is  called  the  flood-plain. 
In  case  of  great  rivers  draining  interior  continental  basins, 
the  flood-plains  are  very  large.     The  flood-plain  of  the 


AQUEOUS  AGENCIES.  31 

Nile  is  the  whole  land  of  Egypt,  for  without  the  Nile  the 
whole  of  Egypt  would  be  a  desert.  Egypt  is  literally  the 
daughter  of  Mlus.  The  flood-plain  of  the  Mississippi 
extends  from  the  mouth  of  the  Ohio  River  to  the  Gulf — 
its  area  is  30,000  square  miles. 

Now,  since  great  rivers  always  rise  in  mountain-regions, 
and  since  the  general  rain-erosion  in  such  regions  is  very 
great,  it  is  evident  that  in  flood  seasons  they  gather  abun- 
dant sediment,  and,  when  these  muddy  waters  overflow, 
the  checking  of  velocity  causes  abundant  deposit  all  over 
the  flood-plain.  With  every  flood  this  deposit  is  renewed, 
and  the  stratum  becomes  thicker.  Thus,  the  level  of  the 
flood-plain  is  built  up  by  sedimentary  deposit,  without 
limit.  In  the  Mississippi  River  the  flood-plain  deposit  is 
about  fifty  feet  thick,  in  the  Nile  it  is  forty  to  eighty 
feet  thick. 

Time. — On  the  flood-plain  of  the  Nile  stand  the  oldest 
monuments  of  civilization  in  the  world.  One  of  these 
(the  statue  of  Rameses  II),  supposed  to  be  3,000  years 
old,  has  been  covered  about  the  base  with  sediment  nine 
feet  deep.  The  whole  thickness  of  the  Nile  sediment  at 
this  point  is  forty  feet — ^nine  feet  in  3,000  years  would 
make  forty  feet  in  13,330  years  as  the  age  of  the  Nile 
deposit.  This  is,  of  course,  but  a  rough  estimate.  The 
rate  may  not  have  been  uniform.  But  in  any  case  the 
whole  time  belongs  to  the  present  geological  epoch. 

Levees,  Natural  and  Artificial. — In  rivers  which 
regularly  flood  their  plains  we  always  flnd  a  sort  of  em- 
bankment on  either  side  near  the  river,  higher  than  the 
rest  of  the  flood-plain,  and  consisting  of  coarser  material. 
This  is  called  the  natural  levee  (Fig.  11,  I,  /).  When  the 
river  is  at  full  flood,  e,  e,  the  whole  flood-plain  is  covered, 
but  at  half  flood  d,  d,  d,  it  is  often  divided  into  three 
streams,  viz.,  the  river  channel  and  the  back  swamp  on 
either  side.  The  cause  of  the  natural  levee  may  be  explained 
thus  :  The  whole  flood-plain  is  covered  with  water  moving 


32 


DYNAMICAL  GEOLOGY. 


slowly  seaward.     Through  the  middle  of  this  compara- 
tively still  water  runs  the  swift  current   of  the   river- 


Fig.  11.— Ideal  section  of  a  flooding  river,    a,  a,  a,  a,  original  bed  ;  6,  6,  flood- 
plain  ;  l^  I,  natural  levees  ;  c,  low  water  ;  d,  d,  d,  half  flood  ;  e,  e,  e,  full  flood. 

channel.  Now,  on  the  two  sides  of  this  swift  current, 
just  wherQ  it  comes  in  contact  with  the  stiller  water,  and 
is  checked  by  it,  there  will  be  a  line  of  abundant  and 
coarser  sediment. 

Artificial  Levees. — Natural  levees  can  not  restrain 
the  floods  of  rivers,  since  they  are  made  by  such  floods. 
By  deposit,  the  bed  of  the  river,  the  natural  levees,  and 
the  back  swamp,  all  rise  together,  maintaining  their  rela- 
tive level.     If,  therefore,  we  desire  to  restrain  the  floods 


Fig.  12.— Ideal  section  of  a  river-bed  and  plain  which  was  built  ud  naturally  for  a 
time  and  then  restrained  by  artificial  levees,  I,  I. 


and  reclaim  the  flood-plain,  we  must  build  artiflcial  levees 
upon  the  natural  ones.  This  interference  modifies  greatly 
the  phenomena  of  deposit.  The  river  continues  to  build 
lip  its  bed  as  before,  and  would  in  time  again  flood  as 
before,  if  the  levees  were  not  built  up  higher  from  time 
to  time.  The  flood-plain,  however,  no  longer  receives 
deposit.  Therefore  the  river-bed  being  raised  by  deposit, 
and  the  levees  by  man,  the  river  finally  runs  on  the  top 
of  an  embankment,  which  rises  ever  higher  above  the  sur- 


AQUEOUS  AQENCIES.  33 

rounding  plain,  and  the  danger  from  accidental  breakage 
of  the  levee  is  ever  greater  (Fig.  12).  It  is  said  that  the 
river  Po,  from  this  cause,  now  runs  above  the  tops  of  the 
houses  on  the  plain. 

4.  Deltas. 

The  flood-plain  of  a  river  may  be  divided  into  two 
parts,  viz.,  the  river-swamp  and  the  delta.  The  river- 
swamp  is  that  part  of  the  flood-plain  which  was  land- 
surface  when  the  river  began  to  run,  and  has  been  noised 
only  a  little  by  deposit.  The  delta  is  that  part  of  the 
flood-plain  which  has  been  reclaimed  hy  the  river  from  the 
empire  of  the  sea.  The  river  has  dumped  sediment  into 
the  sea  or  lake,  until  it  filled  it  up  and  made  a  certain 
amount  of  land.  This  made  land  is  the  delta.  For  ex- 
ample. Upper  Egypt  is  the  river-swamp  ;  Lower  Egypt, 
from  Cairo  seaward,  is  the  Delta.  The  flood-plain  of  the 
Mississippi,  from  the  mouth  of  the  Ohio  to  about  Baton 
Rouge,  is  river-swamp  ;  thence  to  the  Gulf  it  is  delta. 

A  delta  may  be  otherwise  defined  as  an  area  of  flat  land 
at  the  mouth  of  rivers,  usually  of  more  or  less  triangular 
shape,  over  which  the  river  runs  by  inverse  ramification, 
emptying  by  many  mouths.  The  point  where  the  river 
commences  to  divide  is  the  head  of  the  delta.  The  area 
of  some  deltas  is  very  great.  The  delta  of  the  Nile  is 
10,000  square  miles,  the  delta  of  the  Mississippi  is  14,000 
square  miles,  and  the  common  delta  of  the  Ganges  and 
the  Brahmapootra  is  20,000  square  miles.  The  form 
of  the  Mississippi  delta  is  very  irregular.  It  runs  out 
into  the  Gulf  as  a  narrow  tongue  fifty  miles  long,  and 
separated  from  the  Gulf  only  by  low,  narrow  embank- 
ments, which  are  continuations  of  the  natural  levees 
(Fig.  13). 

Deltas  are  not  formed  by  all  rivers,  but  only  by  those 
which  empty  into  tideless,  or  nearly  tideless,  waters. 
Streams  running  into  pools,  ponds,  lakes,  and  rivers  run- 

Lb  Conte,  Geol.  3 


34 


DYNAMICAL   GEOLOOY. 


ning  into  landlocked  seas,  make  deltas  ;  but  rivers  emp- 
tying into  strongly  tidal  seas  have  wide,  bay-like  mouths 
or  estuaries.  The  strong  tides  and  waves  not  only  carry 
away  the  sediment  brought  down  and  prevent  land-mak- 
ing, but  cut  away  and  enlarge  the  mouths  of  the  rivers. 
Thus,  in  this  country,  all  the  rivers  emptying  into  the 
Great  Lakes  or  into  the  Gulf  of  Mexico  (where  the  tides 


Fig.  13.— Mississippi  delta. 


are  very  small)  make  deltas,  while  all  emptying  into  the 
Atlantic  or  Pacific  have  estuaries.  So,  in  Europe,  all  the 
rivers  emptying  into  the  Mediterranean  Sea,  the  Black 
Sea,  the  Caspian  Sea,  the  North  Sea,  and  the  Baltic  Sea, 
form  deltas,  while  those  emptying  into  the  Atlantic  have 
estuaries.  The  Ganges  (Fig.  14)  seems  to  be  an  excep- 
tion to  this  rule  ;  for  it  makes  a  great  delta,  although  the 
tides  in  the  Bay  of  Bengal  are  strong.  The  cause  of  this 
is  the  prodigious  quantity  of  mud  brought  to  the  sea  by 
the  Ganges.     Two  opposite  agencies  are  at  work  at  the 


AQUEOUS  AGENCIES, 


35 


mouths  of  rivers,  viz.,  the  river  bringing  sediment  and 
making  land,  and  the  sea  carrying  it  away  and  destroying 
land.  If  the  former  prevails,  a  delta  is  formed ;  if  the 
latter,  an  estuary. 


Fig.  14.— Delta  of  Ganges  and  Brahmapootra.    (From  De  la  Beche.) 


Mode  of  Formation. — We  are  apt  to  imagine  that  we 
can  not  observe  these  phenomena  except  by  becoming 
travelers.  On  the  contrary,  we  may  observe  them  in  every 
little  stream  emptying  into  a  pond.  In  every  such  case 
we  shall  observe  a  sand-flat  over  which  the  stream  runs 
in  many  rills,  that  often  change  their  position.  Such  a 
sand-flat  is  a  delta.  If  we  watch  the  process,  we  shall  see 
that  the  stream  before  entering  the  pond  carries  sedi- 
ment, perhaps  is  muddy.  As  soon  as  it  strikes  the  still 
water,  it  spreads  out  in  all  directions,  the  velocity  is 
checked,  the  sediment  falls,  the  bottom  is  built  up  to  the 
surface,  and  the  delta  commences.  The  sand  or  mud  is 
now  carried  over  the  delta,  and  dumped  beyond.  Thus 
the  delta  grows  from  day  to  day  (Fig.  lb,  a).     The  stream. 


36 


DYNAMICAL  GEOLOGY. 


as  it  runs  over  the  sand-flat,  is  often  choked  with  its  own 
deposit,  and  compelled  to  seek  new  channels  by  dividing. 
In  the  figure  we  have  represented  the  case  of  a  torrent, 
carrying  coarse  sediment,  rushing  down  a  steep  slope  into 
a  lake  or  pond.  In  such  a  case  the  sediment  falls  quickly, 
the  strata  will  be  irregular  and  highly  inclined  ;  but,  in 
the  case  of  great  rivers  carrying  sediments  for  long  dis- 


.  15.— Ideal  map  (a)  and  section  (6),  showing  the  formation  of  a  delta. 


tances,  the  coarse  material  is  all  dropped  higher  up,  and 
that  which  reaches  the  sea  is  very  fine,  and  therefore  sinks 
slowly.  Hence  in  great  deltas  the  stratification  is  nearly 
horizontal. 

Again,  the  figure  plainly  shows  that,  if  this  process 
goes  on,  the  lake  or  pond  will  be  entirely  filled.  All 
mountain-lakes  are  being  rapidly  filled  in  this  way,  and  a 
little  close  observation  is  sufficient  to  show  that  all  high 
mountain-regions,  like  the  Sierra  or  Colorado  mountains, 
are  full  of  marshes  and  meadows  which  are  extinct  lakes. 

Age  of  Deltas. — All  deltas  are  growing.  The  rate  of 
growth  in  some  cases  has  been  observed.  The  delta  of 
the  Po  has  advanced  twenty  miles  into  the  Adriatic  Sea 
since  Roman  times,  for  the  town  of  Adria,  then  a  sea- 
port, is  now  twenty  miles  inland.  The  delta  of  the  Rhone 
has  grown  thirteen  miles  in  the  Christian  era.     The  Mis- 


AQUEOUS  AGENCIES,  87 

sissippi  delta  is  pushing  seaward  more  rapidly  than  any 
other,  evidently  because  it  pushes  along  narrow  lines.  It 
is  advancing  now  at  the  rate  of  three  hundred  and  thirty 
feet  per  annum,  or  a  mile  in  sixteen  years,  or  six  miles 
per  century.  The  age  of  deltas  can  not,  however,  be  got 
in  this  way,  because  the  river  often  changes  its  mouth, 
and  dumps  its  freight  now  here,  now  there,  along  the 
whole  water-front  of  its  delta.  The  age  or  time  is  usually 
estimated  by  getting  the  cubic  voliune  of  the  delta,  and 

dividing  this  by  the  annual  mud-discharge  \T  =  - — -r  ). 

But  without  more  accurate  observations  than  have  yet 
been  undertaken,  these  estimates  are  not  entitled  to  much 
confidence. 

5.  Estuaries, 

The  wide  mouths  of  certain  rivers  are  called  estuaries. 
We  have  already  explained  why  some  rivers  have  estuaries 
and  some  make  deltas.  All  the  rivers  running  into  the 
Atlantic  and  Pacific  Oceans  have  estuaries,  because  the 
tidal  currents  are  stronger  in  carrying  away  than  the  river 
in  bringing  down  and  depositing  sediments.  The  Bay  of 
Fundy,  the  Hudson  River  to  near  Albany,  the  Delaware 
and  Chesapeake  Bays,  Albemarle  and  Pamlico  Sounds, 
the  Bay  of  San  Francisco,  and  the  Lower  Columbia  River 
are  estuaries.  So  also  are  the  wide  mouths  of  the  Amazon 
and  La  Plata  Rivers.  So  also  the  firths  of  Scotland  and 
the  fiords  of  Norway.  The  velocity  and  therefore  erosive 
power  of  tides  in  estuaries  is  sometimes  enormous.  The 
trumpet-shaped  mouth  of  the  river  takes  in  a  large  mass 
of  the  tide-wave.  As  this  passes  up  it  is  compressed  into 
a  narrower  channel,  and  therefore  rises  higher  and  rushes 
with  increasing  velocity.  In  the  upper  part  of  Bristol 
Channel  the  tide  rises  forty  feet  ;  in  the  Bay  of  Fundy, 
sixty  feet ;  in  Puget  Sound,  twenty  feet.  If  the  water  be 
shallow  and  the  resistance  to  advance  great,  the  tide  rises 


38  DYNAMICAL   GEOLOGY. 

into  a  breaker,  which  advances  at  a  rate  sometimes  twenty 
miles  an  hour.  It  is  evident  that  the  erosive  or  land- 
destroying  power  of  such  currents  is  enormous.  What- 
ever is  thus  gathered  in  its  upward  course,  together  with 
whatever  is  brought  down  as  sediment  by  the  river,  is 
all  carried  by  the  ebb-tide  out  to  sea,  and  therefore  lost 
to  the  land.  In  fact,  of  all  places  along  the  water-front, 
the  mouths  of  rivers  are  the  most  vulnerable  to  the  attacks 
of  the  sea.* 

Deposits  at  the  Mouths  of  Rivers. — The  retreating 
tide  carries  away  to  the  sea  both  what  is  gathered  by  the 
advancing  tide  and  what  is  brought  down  by  the  river. 
Therefore  the  estuary  is  scoured  out,  rather  than  receives 
deposit.  Yet,  in  certain  sheltered  coves — such  as  repre- 
sented in  Fig.  17,  at  a  and  b — stratified  deposits  will  often 
be  found.  These  are  peculiar.  They  consist  of  an  alterna- 
tion of  fresh-water,  brackish-water,  and  salt-water  deposits, 
known  each  by  the  shells  which  they  contain.  The  reason 
is  this  :  In  dry  seasons  these  coves  are  occupied  by  salt 
water  only,  and  in  flood  seasons  by  fresh  water  only. 

Also  along  the  water-front  of  deltas  some  parts  will 
be  receiving  sediments  from  the  river,  while  other  parts, 
receiving  no  such  sediments,  will  be  inhabited  by  marine 
animals.  With  the  shifting  of  the  mouth  of  the  river, 
these  latter  may  be  again  covered  with  river  sediments. 
Therefore,  in  all  deposits  made  at  the  mouths  of  rivers 
there  will  be  an  alternation  of  fresh-  and  salt-water  de- 
posits. And,  conversely,  stratified  deposits,  coyisisting  of 
such  alternations,  wherever  found,  are  judged  by  geologists 
to  have  been  formed  at  the  mouths  of  ancient. rivers, 

6.  Bars. 

The  formation  of  bars  is  an  admirable  illustration  of 
the  laws  of  sediment-laden  currents.     Bars  are  formed  at 

*  Estuaries  are  also  often  formed  by  subsidence  of  the  land.  They 
are  then  the  drowned  lower  courses  of  rivers. 


AQUEOUS  AGENCIES.  39 

fche  mouths  of  all  rivers  by  the  fan-like  spreading  of  the 
currents  and  the  consequent  checking  of  the  velocity  by 
contact  with  still  water  of  the  sea  or  lake.  They  are  usu- 
ally of  semicircular  or  horseshoe-like  form,  as  shown  in 
Figs.  16,  17.  In  rivers  forming  deltas  (Fig.  16)  this  is 
the  only  bar  ;  but  in  rivers  forming  estuaries  (Fig.  17) 
there  are  two  bars — one  at  the  mouth  of  the  estuary,  and 
the  other  at  its  head.  The  bar  at  the  mouth  is  formed 
in  the  usual  way,  by  the  spreading  of  the  current  of  the 
outgoing  tide,  the  consequent 
checking  of  its  velocity,  and  de- 
{..•.^:3v?^^s,  posit  of  its  sediment.  It  has, 
^      V*    '^      therefore,  the  usual  semicircular 

~ .^^    if      or   horseshoe   form.      There  are 

y-^^^ki«  usually  passes  or  deeper  channels 

^  through  which  the  tides  ebb  and 

Fig.  16.  floW. 

The  bar  at  the  head  of  the 
estuary  or  bay  is  formed  by  the  meeting  of  two  opposing 
sediment-laden  currents,  viz.,  the  up-flowing  tide  and  the 
down-flowing  river.  The  meeting  of  these  at  every  tide 
makes  still  water  at  that  place,  and  the  sediments  from 


I 


Pig.  Vi 


both  are  dropped  there.  In  fact,  at  the  head  of  the 
estuary  there  are  three  associated  phenomena,  all  pro- 
duced by  the  meeting  of  these  opposing  currents  :   1. 


40  DYNAMICAL  GEOLOGY. 

The  backing  up  of  the  river-water  by  the  tides  causes  it 
to  overflow.  There  is,  therefore,  here  a  more  or  less  ex- 
tensive marshy  or  swampy  flood-plain.  2.  The  river  here 
not  only  forms  a  bar,  but  also  a  more  or  less  extensive 
flood-plain  deposit.  3.  The  river  winds  tortuously  and 
in  many  channels  through  the  soft,  marshy  soil,  forming 
many  marshy  isles.  These  facts  are  shown  in  Fig.  17. 
In  fact,  we  have  here  many  of  the  phenomena  of  a  river- 
delta.  The  Hudson  Eiver,  for  example,  is  an  estuary,  one 
hundred  and  twenty  miles  long.  The  tide  runs  up  and 
meets  the  river-current,  and  makes  still  water  about 
twenty  miles  below  Albany.  At  this  point  is  the  bar. 
At  this  point  also  is  an  extensive  marshy  overflow-land, 
through  which  the  river  winds  its  tortuous  course.  The 
same  phenomena  are  seen  at  the  head  of  the  Bay  of 
San  Francisco.  The  river  here  winds,  by  many  tortuous 
channels  with  islands  between,  through  an  extensive 
marsh  (tule-lands).  The  bar  is  also,  of  course,  found 
here. 

Removal  of  Bars. — If  a  bar  be  scraped  away,  it  will 
be  re-formed  by  the  same  agencies  which  originally  formed 
it.  Only  constant  dredging  can  improve  it.  If  the  river- 
channel  be  contracted  by  dikes  so  as  to  increase  the  ve- 
locity of  the  current,  it  will  indeed  scour  out  the  bar,  but 
the  latter  will  again  form  at  a  new  point  of  equilibrium  a 
little  lower  down.  In  rivers  forming  deltas  the  bar  has 
been  successfully  removed,  in  some  cases,  by  means  of 
jetties  extending  beyond  the  mouth  of  the  river  into  the 
sea  or  gulf.  The  now  swifter  current  scours  out  the  bar, 
and  the  sediment  is  delivered  in  deep  water,  where  it 
must  deposit  a  long  time  before  the  bar  is  re-formed. 
The  most'  remarkable  examples  of  such  improvement  of 
bars  are  at  the  mouth  of  the  Danube  by  the  Austrian 
Government  and  at  the  mouth  of  the  Mississippi  by  the 
United  States  Government. 

We  have  now  traced  the  agency  of  rain  and  rivers  from 


AQUEOUS  AGENCIES.  41 

mountains  to  sea.  In  fact,  in  the  phenomena  of  estuaries 
and  bars,  we  have  already  a  cooperation  of  rivers  and 
sea.  This  brings  us  very  naturally  to  the  next  head,  viz.. 
Agency  of  the  Ocean, 

Section  II. — The  Ocean. 

Waves  and    Tides. 

The  ceaseless  beating  of  waves  on  an  exposed  shore  can  ^ 
not  fail  to  impress  the  observer  as  a  powerful  erosive 
agent.  Tides  assist  the  waves,  not  only  by  creating  pow- 
erful currents  in  all  bays,  inlets,  and  estuaries,  as  already 
explained,  but  also  by  lifting  the  sea-level  and  therefore 
presenting  new  points  of  attack.  As  in  the  case  of  rivers 
the  erosive  power  is  greatly  increased  by  the  sand,  gravel, 
and  pebbles  carried  by  the  current,  so  in  the  case  of  waves 
the  sand,  gravel,  shingle,  and  rock-fragments  torn  from 
the  cliffs  are  taken  up  again  and  hurled  back  with  vio- 
lence, and  become  the  chief  agents  of  further  erosion. 
Although,  however,  so  incessant  and  violent  in  action 
and  so  conspicuous  in  effects,  yet,  being  confined  wholly 


Pig.  18.— View  of  incnne<l  stratji,  with  faces  exi)08e<l.r.()  waves. 

to  the  shore-line,  the  aggregate  effect  of  wave-erosion 
is  far  less  than  that  of  the  universal  erosion  of  rain  and 
rivers. 


42  DYNAMICAL   QEOLOQT, 

Resulting  Forms. — It  is  interesting  to  trace  the 
forms  of  coast-lines  to  their  causes.  If  the  country-rock 
be  stratified,  and  the  strata  dip  toward  the  sea  so  as  to 
present  their  faces  to  the  waves,  then  the  erosion  will  be 
slower  and  the  coast-line  comparatively  even  (Figs.  18, 
20,  a).  If,  on  the  contrary,  the  strata  be  level  and  the 
waves  act  on  the  edges  (Fig.  19),  then  the  cliff  will  be 


Pig.  19.— Section  view  of  level  strata,  a  and  6,  with  edges  to  waves. 

undermined,  overhanging  tables  will  fall  from  time  to 
time,  and  the  erosion  will  be  rapid.  Finally,  if  the  edges 
of  vertical  or  inclined  strata  be  turned  toward  the  waves 
(Fig.  20,  i),  then  the  coast-line  will  be  deeply  dissected. 


Pio.  20— Map  view  of  inclined  strata,  dipping  northward,  as  shown  by 
a,  faces  to  waves  ;  6,  edges  to  sea. 

i.  e.,  composed  of  alternate  headlands  and  inlets.  In  these 
inlets,  the  waves,  gathering  force  as  they  are  pressed  into 
narrower  channels,  beat  with  prodigious  force. 

Again  :  since  waves  and  tides  act  only  on  the  shore-line 
as  high  as  they  can  reach  on  the  one  hand,  and  as  deep  as 
they  can  touch  bottom  and  form  breakers  on  the  other,  it 
is  evident  that  they   act    as  a  horizontal   saw,  cutting 


AQUEOUS  AGENCIES. 


43 


down  the  land  a  little  below  the  sea-level.  Hence,  along 
a  shore-line  which  has  suffered  much  from  beating  waves, 
we  are  apt  to  find  first  a  steep,  perhaps  overhanging  cliff  ; 
then  a  level,  submarine  plateau  ;  and  then,  as  we  go  far- 
ther, a  sudden  falling  off  to  deep  water.     In  Fig.  21,  the 


Fie.  21. 


-Ideal  section  view  of  submarine  plateau  and  shore-cliff  :  I,  sea-level ;  a,  5, 
submarine  plateau  ;  s,  present,  and  *',  the  original,  shore-line. 


dotted  line  shows  the  original  configuration  of  land  and 
position  of  the  shore-line.  Such  level  plateaus,  termi- 
nated by  cliffs,  are  often  found  far  inland.  In  some  cases, 
though  not  in  all,  they  indicate  old  sea-cliffs. 

Nearly  all  shore-lines  are  receding  under  the  incessant 
action  of  waves  and  tides,  but  the  rate  is  very  different 
in  different  places.  As  rain-erosion  is  concentrated  on 
certain  lines,  giving  rise  to  surface  inequalities,  such  as 
gorges,  ravines,  canons,  etc.,  so  wave  and  tide  erosion 
give  rise  to  nearly  all  the  inequalities  of  coast-line.  The 
general  form  of  continents,  and  their  largest  inequalities, 
are  doubtless  due  to  other  (i.  e.,  continent-making  and 
mountain-making)  causes  ;  but  all  the  promontories,  har- 
bors, bays,  etc.,  are  due  to  ocean-erosion.  As  land  scen- 
ery is  due  mainly  to  rain  and  river  erosion,  so  sea-shore 
scenery  is  due  mainly  to  sea-erosion.  Every  projecting 
promontory  will  usually  be  found  to  consist  of  hard  rock, 
and  every  indentation  is  determined  either  by  the  soft- 
ness of  the  rock  or  else  by  the  mouth  of  a  river  giving 
entrance  to  powerful  tidal  currents. 

Examples  are  found  on  every  coast.  In  our  own  coun- 
try the  rocky  shores  of  New  England  everywhere  show 
the  wasting  action  of  waves.     Farther  south  the  coast  is 


44  DYNAMICAL   GEOLOGY. 

wasting  in  some  places  and  gaining  in  others  ;  for,  as  we 
shall  see  hereafter,  waves  and  tides  may  7nake  as  well  as 
destroy  land. 

In  Europe  examples  are  more  numerous  and  striking, 
and  have  been  more  carefully  studied.  The  rushing  tide 
through  the  English  Channel  and  Dover  Strait  has 
greatly  enlarged  and  is  still  enlarging  the  channel.  The 
eastern  coast  of  England  is  now  being  eaten  away  at  the 
rate  of  from  three  to  five  feet  per  annum.  The  church 
of  the  Reculvers,  which  stands  near  the  mouth  of  the 
Thames,  and  for  many  centuries  has  been  a  landmark  for 
ships  entering  that  port,  stood,  in  the  time  of  Henry 
VIII,  one  and  a  half  miles  inland,  on  a  high  cliff.  It  is 
now  on  the  sea-margin,  and  would  have  long  ago  fallen 
into  the  sea  if  it  had  not  been  saved  by  an  artificial  sea- 
wall. Many  isles  in  the  German  Ocean  have  entirely  dis- 
appeared in  this  way.  Heligoland  is  fast  going,  and 
already  almost  gone. 

The  western  coast  of  England,  Ireland,  and  Scotland 


Fig.  22. 


is  wasting  less  rapidly  at  present,  but  only  because  noth- 
ing but  hard  rock  is  left.     The  deeply  dissected  coast- 


AQUEOUS  AGENCIES. 


45 


lines,  with  high  promontories,  separated  by  deep  inlets, 
show  the  waste  they  have  suffered  in  previous  geological 
times.  As  we  go  north,  the  evidences  of  destruction 
become  more  and  more  conspicuous.  To  the  north  of 
Scotland,  among  the  Hebrides,  Orkneys,  and  Faroe  Isl- 
ands, are  found  groups  of  bare,  wave- worn  rocks,  standing 
in  the  midst  of  the  sea,  mere  skeletons  of  once  fertile 
islands  (Fig.  22).      • 

But  all  these  effects,  viz.,  boldness  of  the  headlands, 
the  depth  of  the  inlets,  the  intricacy  of  coast-dissection, 
reach  their  highest  point  on  the  coast  of  Norway.  Any 
good  map  of  this  country  (see  Fig.  23)  shows  that  the 
whole  coast  consists  of  alternate  promontories  and  inlets. 
The  promontories  are  rocky  head- 
lands, 2,000  to  3,000  feet  high,  and 
the  inlets  run  50  to  100  miles  inland. 
Such  deep  inlets,  separating  high 
headlands,  are  called /or^s.  Closer 
inspection  shows  a  line  of  islands  off 
the  coast.  These  are  rocky  islands 
2,000  to  3,000  feet  high,  and  of  hard- 
est granite.  These  granite  isles  are 
probably  the  aa:is  of  the  Scandina- 
vian mountains — in  fact,  of  the  Scan- 
dinavian Peninsula.  If  so,  then  it 
would  seem  that  the  whole  western 
slope  of  these  mountains  has  been 
swept  away,  that  the  sea  has  already 
broken  through  the  axis  or  backbone,  and  is  now  gnaw- 
ing among  the  ribs  on  the  eastern  flank.  On  nearly  all 
bold  and  severely  beaten  coasts  we  find  such  off-shore 
islands,  which  are  the  fragments  of  a  former  coast-line. 

The  present  form  of  the  ^N^orway  coast,  however,  is  not 
wholly  due  to  sea-erosion,  but  also  largely,  as  we  shall 
show  hereafter,  to  subsidence.  Yet,  as  Norway  is  per- 
haps the  oldest  part  of  the  European  continent,  we  have 


Fig.  23.— Map  of  Norway 
coast,  showing  the  dis- 
sected coast-line  and  isl- 
ands off  shore. 


46  DYNAMICAL   GEOLOGY. 

probably  not  exaggerated,  in  what  is  said  above,  the 
ravages  it  has  suffered  from  its  ancient  enemy,  the 
sea. 

Transportation  and  Deposit. — The  lifting  power  of 
waves  is  immense,  often  taking  up  rock-fragments  of 
many  tons  weight  and  hurling  them  with  violence  against 
the  shore-line  ;  but  they  usually  carry  only  a  very  short 
distance.  Jnder  certain  conditions,  however,  waves 
may  transport  materials  for  many  hundreds  of  miles. 
Thus,  on  account  of  the  trend  of  the  Atlantic  coast  and 
the  prevalence  of  north  winds,  the  coast  material  is  cast 
up  on  shore  and  falls  off  a  little  southward  with  every 
wave.  Thus,  shore-sands  creep  southward  slowly,  even 
to  the  point  of  Florida,  although  the  coast-rock  of  Flor- 
ida is  all  limestone.  So,  also,  the  shore-sands  of  Lake 
Michigan  are  carried  southward  by  wave-action,  and 
accumulate  about  Chicago. 

Though  waves  are  usually  ^/estructive  rather  than  co7i- 
structive,  yet  they  often  add  to  the  land  along  shore- 
lines by  deposits.     Such  deposits  are  very  characteristic  : 

1.  They  are  usually  coarse  material  and  thoroughly 
water-worn — i.  e.,  round- gramed  sand,  gravel,  or  shingle. 

2.  The  lamination  is  often  highly  inclined  and  irregular. 

3.  They  are  often  affected  with  ripple-marks  (Fig.  24). 


Fig.  24.— Ripple-marks. 

4.  They  are  often  impressed  with  tracks  of  animals  and  with 
rain-drops.  Now,  all  these  marks  are  found  in  rocks  far 
inland  and  high  up  the  slopes  of  mountains.     We  can  thus 


AQUEOUS  AGENCIES.  47 

often  recognize  the  old  shore-lines  of  previous  geological 
epochs. 

Oceanic  Currents, 

The  earth  is  covered  with  two  oceans,  an  atmospheric 
and  an  aqueous.  The  former  covers  the  whole  of  it,  fifty 
or  more  miles  deep  ;  the  latter  covers  three  quarters  of  it, 
three  miles  deep.  On  the  surface  of  the  one  we  swim  and 
sail,  on  the  bottom  of  the  other  we  crawl.  Both  of  these 
oceans  are  in  constant  circulation  in  every  part.  The  cur- 
rents in  the  one  are  winds,  in  the  other  oceanic  streams. 
The  cause  of  circulation  and  the  general  directions  of  the 
currents  are  also  the  same.  There  is,  indeed,  some  dif- 
ference of  opinion  as  to  the  immediate  cause  of  oceanic 
currents  ;  but  there  can  be  no  doubt  that,  directly  or  indi- 
rectly, they,  like  winds,  are  caused  by  difference  of  tem- 
perature between  the  equator  and  the  poles.  In  both 
cases,  too,  there  are  disturbing  causes  which  complicate 
the  result.  In  the  one  case,  local  variations  of  tempera- 
ture and  extreme  mobility  of  the  medium  ;  in  the  other, 
the  existence  of  unseen  submarine  banks,  and  especially 
of  impassable  barriers,  the  continents.  As  our  sole  object 
is  to  discuss  their  geological  agency,  we  shall  describe  but 
one  of  these  great  oceanic  currents  as  an  example. 

Gulf  Stream. — This  stream  takes  its  origin  in  the  equa- 
torial current  which,  stretching  across  the  Atlantic  from 
the  coast  of  Africa,  strikes  the  wedge-shaped  eastern  point 
of  South  America  and  divides  north  and  south.  The 
larger  northern  branch  runs  along  the  coast  of  South 
America  into  the  Caribbean,  and  thence  through  the 
Straits  of  Florida  into  the  North  Atlantic.  After  passing 
the  point  of  Florida  it  turns  north,  runs  along  the  coast  of 
the  United  States,  turns  eastward  from  the  coast  of  Lab- 
rador, and,  after  sending  a  branch  toward  the  Arctic  re- 
gion north  of  Europe,  turns  southward  to  join  again  the 
equatorial  current  on  the  coast  of  Africa.     The  amount  of 


48  DYNAMICAL  OEOLOOY. 

water  carried  by  this  ocean-stream  is  probably  greater  than 
that  of  all  the  mvers  in  the  world.  It  is  equivalent  to  a 
stream  fifty  miles  wide,  one  thousand  feet  deep,  and  run- 
ning at  a  rate  of  three  miles  per  hour.  Its  extreme 
Telocity,  where  it  passes  through  the  Straits  of  Florida,  is 
four  to  five  miles  per  hour. 

Oeological  Agency. — Oceanic  streams  run  on  beds  and 
between  banks  of  still  water,  and  therefore,  probably, 
have  no  erosive  agency,  but  they  are  important  agents  in 
the  transportation  and  distribution  of  sediments.  All  the 
debris  of  land-surfaces  are  brought  by  rivers  to  the  water- 
front and  dumped  there.  Tides,  in  their  retreat,  may  take 
these  seaward  a  few  miles,  but  these  also  soon  lose  their 
velocity  and  drop  their  freight.  Were  it  not  for  oceanic 
currents,  therefore,  the  whole  debris  of  land-surfaces 
would  be  dropped  within  thirty  to  fifty  miles  of  the  shore- 
line. As  it  is,  nine  tenths  are  so  dropped.  Marginal 
sea-bottoms  are,  therefore,  the  great  theaters  of  sediment  a- 
tio7i.  Nevertheless,  a  small  portion  of  finest  sediment  is 
carried  within  reach  of  oceanic  currents,  and  by  them 
strewed  broadcast  over  portions  of  deep-sea  bottom. 

We  find  good  examples  of  this  in  the  course  of  the  Gulf 
Stream.  The  sediments  of  the  Amazon  may  be  traced 
from  its  mouth  seaward  for  a  great  distance.  It  is  then 
taken  by  oceanic  currents  and  carried  northward,  and 
much  of  it  deposited  on  the  coast  of  Guiana,  three  hun- 
dred miles  distant,  and  the  remainder  into  the  Caribbean 
Sea.  According  to  Humboldt,  much  sediment  is  carried 
from  the  Caribbean  into  the  Gulf  of  Mexico.  The  stream 
receives  there  also,  possibly,  contributions  from  all  the 
Gulf  rivers,  especially  the  Mississippi,  and  may  deposit 
these  again  along  the  coast  of  Florida  and  the  Bahama 
Islands. 

The  surface  transparency  often  conspicuous  in  oceanic 
currents  is  no  evidence  against  their  carrying  sediments ; 
for  there   is  tliis  dift'erence  between  rivers  and  oceanic 


AQUEOUS  AGENCIES. 


49 


streams  :  In  rivers,  besides  the  general  current,  there  are 
partial  currents,  from  side  to  side  and  up  and  down,  which 
keep  the  stream  turbid  to  the  surface ;  while  ocean-streams, 
running  on  beds  and  between  banks  of  still  water,  have  no 
such  partial  currents.  There  is  nothing  to  prevent  sedi- 
ments settling  exactly  as  in  still  water.  Thus  ocean-cur- 
rents usually  carry  sediments,  if  at  all,  only  in  their  deeper 
parts.  Deep-sea  deposits  are  undoubtedly  of  great  im- 
portance, but  only  recently  have  attracted  much  attention. 
Submarine  Banks. — These  are  formed  by  checking 
the  velocity  of  sediment-laden  currents,  whether  tidal  or 
oceanic.  The  checking  may  be  caused  by  the  meeting  of 
two  opposing  currents,  or  by  the  current  passing  through 


Fig,  25.— Tides  of  the  German  Ocean. 


a  narrow  strait  into  a  ^vide  sea.     In  other  words,  subma- 
rine banks  are  formed  under  the  same  conditions  as  bars ; 

Lb  Conte,  Gbol.  4 


50  DYNAMICAL   OEOLOOV. 

and  bars  at  the  mouths  of  rivers  are,  in  fact,  one  form  of 
submarine  bank. 

The  best  examples  of  banks  formed  by  tidal  currents 
are  found  in  the  North  Sea.  It  is  seen  in  the  map,  Fig. 
25,  that  the  tidal  wave  from  the  Atlantic,  striking  on  the 
British  Isles,  divides  into  two  parts,  one  entering  the  North 
Sea  through  Dover  Strait,  the  other  by  the  Shetland  and 
Faroe  Islands.  That  through  Dover  Strait  runs  swiftly 
through  the  narrowing  channel,  gathering  much  sediment. 
As  soon  as  it  passes  Dover  Strait  it  spreads  fanlike,  its 
velocity  is  checked,  and  it  deposits  sediment.  In  the 
mean  time  the  other  branch,  coming  in  from  the  north, 
meets  the  southern  branch,  and  makes  still  water  at  some 
point,  as  a,  and  deposits  sediment.  Again,  all  the  rivers 
emptying  into  this  sea  from  the  south  form  bars  at  their 
mouths.  To  these  several  causes  are  due  the  numerous 
banks  which  render  the  navigation  of  this  shallow  sea  so 
dangerous.  Banks  are  formed  also  by  oceanic  currents. 
For  example,  the  Gulf  Stream,  passing  through  the  Straits 
of  Florida,  eddies  on  both  sides  and  forms  the  Bahama 
and  Florida  Banks.  Again,  the  Banks  of  Newfoundland 
are  at  least  partly  formed  by  the  Arctic  current  bearing 
icebergs  loaded  with  debris  from  Greenland  (see  pp.  66, 
67),  meeting  the  warm  Gulf  Stream,  whereby  the  bergs 
are  melted  and  their  burden  dropped. 

Land  formed  by  the  Agency  of  Waves. — We  have 
spoken  of  waves  only  as  destroying  land,  but  under  suita- 
ble conditions  they  also/orw  land.  On  submarine  banks, 
however  produced,  islands  are  formed  by  waves.  When 
by  sedimentary  deposit  the  bank  is  built  up  to  near  the 
water-surface,  so  that  the  waves  toucli  bottom  and  form 
breakers,  then  the  bank  is  beaten  up  above  the  surface 
and  forms  islands,  which  continue  to  grow  by  the  same 
agency.  Such  islands  are  always  low,  narrow,  and  long 
in  the  direction  of  the  coast-line.  In  this  way  are  formed 
the  small  islands  which  overdot  the  surface  of  extensive 


AQUEOUS  AGENCIES. 


51 


submarine  banks— also  the  low  islands  about  the  mouths 
ot  estuaries  ana  harbors,  such  as  Sandy  Hook  and  Coney 
Island  about  New  York  _ 

Harbor,  and  the  long 
sand-spits  off  the  shores 
of  shallow  seas,  as,  for 
example,  the  shores  of 
North  Carolina  (Fig. 
26),  and  nearly  the 
whole  southern  Atlan- 
tic coast.  The  debris 
brought  down  by  the 
rivers  to  the  estuaries 
is  carried  by  the  re- 
treating tide  seaward 
and  dropped  near  shore. 
On  the  sea-margin  bank 
thus  formed,  the  waves 
beat  up  long,  narrow 
sand  -  spits,  separated 
only  by  tidal  inlets. 
This  is  the  condition  on 
the  North  Carolina 
coast.  These  barriers 
to  the  retreating  tide 
then  cause  the  estuaries  to  fill  up,  until  they  are  separated 
from  the  mainland  only  by  narrow  tidal  channels.  Thus 
are  probably  formed  the  sea-islands  on  the  South  Carolina 
coast.  Finally,  the  tidal  chp.nnels  may  be  filled  up,  the 
islands  added  to  the  land,  and  the  coast-line  transferred 
seaward. 

Along  nearly  all  coasts  we  find  a  line  of  small  islands. 
These  are  of  two  kinds.  The  one  consists  of  high,  rocky 
islands,  oif  bold  coasts,  as  in  Norway,  Greenland,  etc.; 
the  other  of  low,  sandy  islands,  off  level  coasts,  as  on  the 
southeastern  coast  of  the  United  States.     Those  of  one 


^^^"^'^ 

1 

g^li 

J 

-SS^^^Jj 

1- 

k 

^K^^^m 

^^hIt 

^-^^^= 

^Hv  .^ 

«"= 

^^BB 

9^  ^ 

-.^^^= 

1  c°       - 

^JA 

IMBG^F'  " 

/-/\.M-» 

\^ 

'"^m 

i 

/o 

-    :^= 

l^ 

d 

1      — &- 

1 

^ 



—      ^ 

Fig.  26. — North  Carolina  coast. 


52  DYNAMICAL  GEOLOGY, 

kind  are  formed  by  land-destroying,  of  the  other  kind  by 
land-forming  action  of  waves.  The  one  are  the  scattered 
remnants  of  an  old  coast-line,  the  other  the  beginnings  of 
a  new  coast-line. 


Section  III. — Ice. 

Ice  may  act  either  as  land-ice — glaciers,  or  as  floating 
ice — icebergs. 

Glaciers. 

The  action  of  glaciers  can  not  be  observed  by  every  one 
in  his  own  locality,  since  they  exist  only  in  very  high 
mountains  or  in  high  latitudes  ;  but  the  subject  is  a  very 
fascinating  one,  and  some  knowledge  of  it  gives  additional 
interest  to  mountain-travel. 

The  summits  of  high  mountains,  especially  in  cool, 
moist  climates,  are  not-  only  covered  with  perpetual  snow, 
but  from  this  snow-cap  there  extend  down  the  valleys, 
far  below  the  region  of  perpetual  snow,  solid  masses  of 
ice,  which  are  in  continual,  slow  motion  downward. 
These  valley  prolorigations  of  the  snoiu-ca'ps — these  moving 
masses  of  ice,  these  ice-streams — are  called  glaciers. 

All  mountain-peaks  and  mountain-ridges  are  trenched 
on  the  sides  with  radiating  or  transverse  valleys.  Now, 
if  we  imagine  such  a  peak  or  ridge  to  be  covered  deeply 
with  snow  and  ice,  and  if  we  imagine,  further,  that  ice  is 
a  stiffly  viscous  substance  like  pitch,  so  that  under  the 
heavy  pressure  of  the  thick  mass  it  runs  slowly  down  the 
slope  of  the  valleys  to  half-way  down  the  mountain,  then 
we  have  the  condition  of  things  as  they  exist  in  the  Alps, 
or  in  any  other  glaciated  region.  In  most  mountains  the 
valleys  are  occupied  by  rivers  ;  in  glacial  regions  they  are 
occupied  in  their  upper  parts  by  glaciers,  and  in  their 
lower  parts  by  rivers.     As  rivers,  so  glaciers,  have  their 


AQUEOUS  AGENCIES.  53 

tributaries,  only  the  tributaries  of  glaciers  are  far  less 
numerous  than  those  of  rivers. 

We  have  said  that  glaciers  are  in  continual,  slow  motion 
down  the  valley ;  yet  in  temperate  climates  they  do  not 
reach  the  sea — they  do  not  reach  beyond  a  certain  point, 
called  the  lower  limit  of  glaciers.  Under  constant  condi- 
tions the  snout  of  the  glacier  remains  unmoved  at  this 
place,  even  though  the  glacier  is  in  constant  current- 
motion.  This  apparent  anomaly  may  be  explained  tnus  : 
The  glacier  may  be  regarded  as  under  the  influence  of 
two  opposite  forces.  Gravity  urges  it  by  slow  motion 
downward,  and,  if  this  acted  alone,  the  glacier  would 
run  into  the  sea.  But  the  ice  is  constantly  melted,  more 
and  more,  as  the  glacier  presses  downward  into  warmer 
regions ;  if  this  alone  acted,  the  point  of  the  glacier 
would  retreat  to  the  summit-snow.  Now,  where  these 
two  forces — one  tending  to  lengthen,  the  other  to  shorten 
the  glacier — balance  each  other,  is  found  the  lower  limit 
of  the  glacier  where  the  snout  rests  unmoved.  Some- 
times, after  a  succession  of  cool,  moist  years,  or  a  succes- 
sion of  heavy  snowfall  years,  the  melting  is  less  rapid  or 
the  motion  more  rapid,  and  the  snout  of  the  glacier  will 
slowly  advance,  perhaps  invading  cultivated  fields  and 
overturning  houses.  Sometimes,  on  the  contrary,  from 
more  rapid  melting  or  less  rapid  motion,  the  snout  will 
recede,  strewing  debris  in  its  former  bed.  But,  whether 
the  snout  stands  still,  or  moves  fortuard,  or  moves  back- 
ward, the  matter  of  the  glacier  is  moving  constantly 
downward.  In  this  respect,  glaciers  are  like  rivers  in 
certain  dry  regions.  These  rivers  rise  in  the  mountains, 
run  a  certain  distance,  but  never  reach  the  sea — never 
pass  a  certain  point  where  the  supply  is  balanced  by 
waste  from  evaporation. 

We  have  said  that  glaciers  reach  far  below  the  line  of 
perpetual  snow.  In  the  Alps,  for  example,  the  lower 
liml-t  of  glaciers  is  5,000  feet  below  the  snow-line.     This 


54 


DYNAMICAL   GEOLOGY. 


AQUEOUS  AGENCIES. 


55 


shows  that  the  mass  of  ice  is  so  great  that,  although  mov- 
ing at  a  rate  of  only  a  few  feet  a  day,  it  may  reach  a  mile 


Fig.  28.— Zermatt  glacier.     (After  Agassiz.; 

below,  and  many  miles  beyond,  the  snow-line  before  it  is 
all  melted.  In  high  latitudes,  where  the  snow-line  comes 
nearer  the  sea-level,  glaciers  not  only  touch  the  sea,  but 
run  far  into  the  sea,  and,  breaking  off,  form  icebergs. 

General  Description  of  a  Glacier. — In  glacial  re- 
gions, where  the  summit  snow-fields  are  large,  every  val- 
ley is  filled  for  a  certain  distance  with  a  glacier.  In  the 
Alps  the  glaciers  are  five  to  fifteen  miles  long,  one  to 
three  wide,  and  two  hundred  to  six  hundred  feet  thick. 
In  the  Himalayas  they  are  twenty  to  forty  miles  long.  In 
the  United  States  (exclusive  of  Alaska)  the  largest  glaciers 
occur  in  Washington.     White  River  glacier,  on  Mount 


56  DYNAMICAL  GEOLOGY, 

Eainier,  Washington,  is  ten  miles  long  and  five  miles  wide. 
On  Mount  Shasta,  California,  glaciers  are  found  five  miles 
long.  In  the  Sierra,  California,  and  in  the  Wind  River 
Mountains,  Northwestern  Wyoming,  small,  imperfect  gla- 
ciers still  linger  in  the  highest  and  shadiest  valleys,  near 
the  summits.  Many  glaciers  are  also  found  in  Norway, 
and  especially  in  Alaska.  But  it  is  only  in  polar  regions 
that  glaciers  are  developed  now  in  such  proportions  as  to 
give  us  any  adequate  idea  of  their  great  importance  as  a 
geological  agent.  Greenland  is  a  land-mass  of  almost  con- 
tinental size,  being  1,200  miles  long  and  600  wide.  It  is 
apparently  completely  covered  with  snow  and  ice,  to  a 
depth  of  2,000  to  3,000  feet.  This  whole  ice-mantle 
moves  bodily  seaward,  and  divides  only  at  the  coast  into 
separate  glaciers,  running  into  the  sea  through  fiords,  and 
there  forming  icebergs.  These  separate  marginal  glaciers 
are  a  mere  fringe  to  the  great  interior  ice-sheet,  and  yet 
many  of  them  are  thirty  to  forty  miles  long  and  many 
miles  wide. 

Greneral  Structure. — As  we  go  from  the  summit  down 
a  glacial  valley,  we  pass  from  ordinary  snow  through 
granular  ice  {neve)  to  the  perfect  ice  of  the  glacier  proper. 
This  glacier-ice,  however,  is  not  clear,  solid  ice,  but 
7nainly  a  white  vesicular  ice,  though  traversed  in  many 
places  by  veins  of  clear,  bb:ie,  solid  ice,  which  gives  the 
whole  a  striped  or  agate-like  appearance.  Moreover,  the 
glacier  is  broken  by  great  transverse  fissures,  which  often 
reach  clear  to  the  bottom  {crevasses),  by  many  marginal 
fissures  along  the  sides,  and  by  smaller,  even  capillary 
fissures,  which  give  it  a  more  or  less  grained  structure. 

As  the  ice  is  constantly  melting  by  the  heat  of  the  sun 
and  air  and  by  contact  with  tlie  rocky  bed,  the  surface  is 
full  of  streams.  These  soon  fall  into  crevasses,  and  find 
their  way  to  the  bottom  and  down  the  glacier-bed  to  tho 
valley  below.  Thus  from  the  snout  of  every  glacier  runs 
a  stream.     The  surface   of  a  glacier  is  not   smooth,  as 


AQUEOUS  AGENCIES.  67 

might  at  first  be  supposed,  but  usually  very  rough.  This 
roughness  is  due  partly  to  rock-fragments  from  the  crum- 
bling cliffs  on  each  side,  as  will  be  presently  explained  ; 
partly  to  the  unequal  melting  of  the  ice  by  the  sun,  pro- 
ducing pinnacles  and  hollows,  as  erosion  produces  hills 
and  valleys  on  land  ;  and  partly  to  the  crevasses.  For 
these  reasons,  the  travel  over  the  surface  is  often  not  only 
difficult  but  dangerous,  especially  as  the  crevasses  are 
often  concealed  by  recently  fallen  snow. 

Moraines  ;  Lateral  Moraines. — On  each  margin  of  a 
glacier,  near  the  bounding  cliffs,  is  found  a  continuous 
pile  of  debris,  consisting  of  earth  and  rock-fragments  of 
all  sizes  up  to  many  hundred  tons  weight.  The  pile  may 
be  twenty  to  thirty  feet  high,  and  is  itself  raised  on  an 
ice-ridge  formed  by  the  protection  of  the  ice  beneath 
from  the  melting  power  of  the  sun.  These  two  marginal 
piles  of  debris  are  called  the  lateral  moraines.  They  are 
formed  by  the  constant  fall  of  rocks  and  earth  from  the 
crumbling  cliffs  on  each  side.  But  as  the  cliffs  are  not 
everywhere  so  steep  that  their  fragments  reach  the  glacier, 
if  the  glacier  were  motionless  the  contributions  would  be 
in  isolated  heaps  only.  But  the  motion  of  the  glacier 
converts  these  separate  contributions  into  a  continuous 
ridge  ;  and,  conversely,  the  continuity  of  the  moraines  is 
a  proof  of  the  motion  of  the  glacier. 

Medial  Moraine. — AVhen  two  tributary  glaciers  unite 
to  form  a  trunk-glacier,  the  two  interior  lateral  moraines 
of  the  tributaries  unite,  and  from  the  angle  between  the 
two  tributaries  will  train  off  as  a  continuous  ridge  of 
debris  along  the  middle  of  the  trunk-glacier  to  its  point. 
This  is  called  a  medial  moraine.  It  is  still  more  indispu- 
table proof  of  the  motion  of  the  glacier,  since  it  is  obvi- 
ously impossible  for  debris  to  reach  the  middle  of  a  glacier 
in  any  other  way.  The  number  of  these  medial  moraines 
will  depend  upon  the  complexity  of  the  glacial  system,  for 
there  will  be  one  for  every  tributary  (Fig.  29).     Even  a 


58  DYNAMICAL   GEOLOOY. 

rocky  island  in  the  middle  of  a  glacier  or  of  a  tributary  will 
give  rise  to  a  separate  train.  Thus,  complex  glaciers, 
with  many  tributaries,  may  be  covered  with  these  trains. 

Terminal  Moraine. — Remembering  that  glaciers  are 
in  constant  motion  and  yet  never  pass  beyond  a  certain 
point,  it  is  evident  that  everything  which  is  carried  by 
the  glacier  must  find  its  resting-place  at  the  foot.  Here, 
then,  we  find  an  enormous,  irregularly  concentric  pile 
of  debris,  the  accumulation  of  ages.  This  is  called  the 
termmal  moraine.  It  is  composed  mainly  of  materials 
carried  on  the  surface  of  the  glacier  (top  moraine),  but 
also  to  some  extent  of  materials  pushed  out  from  beneath 
(ground  moraine). 

The  Motion  of  Glaciers  and  its  Laws. — That  gla- 
ciers are  actually  in  continual  motion  downward  is  proved 
by  the  constant  change  of  position,  in  relation  to  points 
on  the  bounding  cliffs,  of  conspicuous  bowlders  lying  on 
the  surface  of  the  glacier.  From  day  to  day  and  from 
year  to  year  these  are  carried  farther  and  farther  down 
the  valley.  With  a  good  theodolite  the  movement  of 
objects  on  the  surface  may  be  observed  from  hour  to 
hour.  Thus,  not  only  the  fact  and  the  rate  but  the  laws 
of  motion  have  been  determined.  The  rate  of  motion  of 
Alpine  glaciers  is  one  to  three  feet  per  day.  The  average 
rate  of  the  Mer  de  Glace  (Fig.  29)  is  estimated  by  Forbes 
as  about  five  hundred  feet  per  annum.  The  extreme 
length  of  the  glacier  is  ten  miles.  A  stone  fallen  upon 
its  upper  part  would  find  its  resting-place  on  the  terminal 
moraine  only  at  the  end  of  one  hundred  years.  Every- 
thing upon  or  beneath  or  within  the  substance  of  the 
glacier  is  finally  deposited  there.  A  striking  and  sad 
illustration  of  this  is  found  in  the  fact  that,  in  several 
cases,  the  mangled  remains  of  adventurous  climbers,  who 
have  fallen  into  crevasses  and  perished,  have  appeared, 
after  many  years,  at  the  foot  of  the  glacier. 

Laws  of  Motion. — A  glacier  moves,  not  like  a  solid 


AQUEOUS  AGENCIES. 


59 


body,  all  together,  sliding  on  its  bed,  but  exactly  like  a 
stiffly   viscous  body.     In  other  words,  the  motion  of  a 


Ftg.  29. — Mer  de  Glace,  with  its  tributaries  and  its  moraines. 

glacier  is  a  current-motion.      Like  a  river,  it  not  only 
slides  on  its  bed,  but  also  the  different  parts  move  with 


GO  DYNAMICAL  OEOLOQY. 

different  velocities,  and  therefore  slide  on  each  other. 
This  is  called  differential  motion,  and  is  characteristic  of 
fluid,  as  distinguished  from  solid  motion.  But  the  differ- 
ential motion  of  glaciers  is  not  free,  like  that  of  water, 
but  reluctant,  and  with  much  resistance,  like  that  of  very 
stiff  pitch.  In  small  masses,  and  under  quickly  applied 
force,  it  breaks  like  a  solid  ;  but  in  large  masses,  and 
under  heavy,  slowly  applied  force,  it  behaves  like  a  stiffly 
viscous  fluid. 

Thus,  like  rivers,  glaciers  move  much  faster  in  the 
middle  than  on  the  margins,  and  on  the  top  than  near  the 
bottom.  Like  rivers,  also,  they  move  faster  on  steep  than 
on  gentle  slopes.  Like  rivers,  also,  the  velocity  increases 
with  the  depth  of  the  stream.  For  example,  the  Mer  de 
Glace  is  350  feet  deep,  and  moves  a  foot  and  a  half  per 
day,  while  some  of  the  great  Greenland  glaciers,  2,000  to 
3,000  feet  deep,  with  less  slope,  run  sixty  feet  per  day. 
Like  rivers,  also,  glaciers  conform  to  the  inequalities  of 
their  bed  and  banks,  but,  as  it  were,  reluctantly — i.  e., 
they  conform  to  large  and  gentle  inequalities,  but  not  to 
the  small  and  sharp  ones.  In  a  word,  glaciers,  like  rivers, 
have  their  narrows  and  their  lakes,  their  shallows  and 
their  deeps,  their  cascades  and  their  level  reaches.  They 
are  truly  ice-rivers. 

Glaciers  as  a  Geological  Agent. 

The  structure,  the  properties,  and  the  cause  of  the 
motion  of  glaciers  are  questions  of  deepest  interest  to 
the  physicist,  but  it  is  their  geological  agency — i.  e.,  their 
agency  now,  and  still  more  in  former  times,  in  sculptur- 
ing the  earth's  surface — which  chiefly  concerns  the  geol- 
ogist. 

We  have  seen  that  a  glacier  may  be  regarded  as  an  ice- 
river.  Like  rivers,  therefore,  glaciers  erode  their  beds  and 
banks,  transport  materials,  and  make  deposits.     But  in 


AQUEOUS  AGENCIES.  61 

all  these  respects  the  effect  of  their  action  is  very  charac- 
teristic,  and  can  not  be  mistaken  for  that  of  water. 

Erosion. — When  we  remember  the  thickness  and 
weight  of  glaciers,  we  at  once  see  that  they  mnst  rub 
with  great  force  on  their  beds.  But,  like  water,  glaciers 
will  do  but  little  work  without  graving -tools.  Rivers 
erode  mainly  by  means  of  sand,  gravel,  and  pebbles,  car- 
ried along  the  bottom  ;  glaciers,  by  means  of  rock-  frag- 
ments of  all  sizes  carried  between  the  ice  and  the  beds, 
and  often  fixed  by  freezing  in  the  ice.  These  graving- 
tools  are  partly  fragments  broken  off  from  the  bed,  and 
partly  fragments  fallen  on  the  surface,  which  become 
ingulfed  in  crevasses  or  jammed  between  the  sides  of  the 
glacier  and  the  bounding  cliffs.  In  either  case,  by  virtue 
of  the  viscosity  of  the  glacier-ice,  they  find  their  way 
finally  to  the  bottom.  By  virtue  of  the  hardness  and 
stiffness  of  the  ice,  it  tends  to  plane  to  one  level ;  by  its 
smoothness,  it  polishes  ;  by  means  of  its  graving-tools,  it 
scores  in  straight,  parallel  lines  ;  by  virtue  of  its  viscosity, 
it  conforms  to  the  large  and  gentle  inequalities,  giving 
rise  to  smooth,  billowy  surfaces,  called  roclies  moiUonnees. 
Thus  smooth,  billowy  surfaces,  scored  with  straight,  par- 
allel marks,  are  very  characteristic  of  glacial  action.  We 
shall  call  such  surfaces  glaciated  (Fig.  30). 

Transportation. — The  transporting  power  of  running 
water  increases  as  the  sixth-power  of  the  velocity.  Even 
at  this  enormous  rate  of  increase,  blocks  of  stone  of  many 
hundreds  of  tons  weight,  such  as  are  often  found  in  the 
paths  of  glaciers,  would  require,  if  carried  by  water,  an 
almost  incredible  velocity.  But  glaciers  carry  materials 
resting  on  their  surfaces,  and  therefore  of  all  sizes,  with 
equal  ease.  Rock-fragments  of  thousands  of  tons  weight 
are  carried  by  them  and  left  in  their  path  by  retreat. 

Again,  fragments  carried  by  water  are  always  more  or 
less  bruised,  worn,  and  rounded,  while  fragments  carried 
on  the   surface  of  glaciers  are  angular.     Again,  water 


DYNAMICAL  GEOLOGY, 


currents  set  down  blocks  of  stone  in  secure  positions ; 
while  gluciers,  in  their  slow  melting,  often  leave  them 


I  ifH- 


FiG.  30.— Glaciated  surface  with  perched  bowlders.    (After  Geikie.) 


perched  in  insecure  positions,  and  even  sometimes  as  rock- 
ing-stones. 

Deposit. —  Materials  deposited  by  water  are  always 
neatly  sorted  and  stratified  ;  the  materials  deposited  by 
glaciers  in  terminal  moraines  are  a  confused  heap  of 
fragments  of  all  sizes,  from  fine  earth  to  great  blocks  of 
stone,  dumped  together  loithout  sorting.  This  heap  con- 
sists of  two  parts  :  the  one  contributed  from  above,  the 
other  pushed  out  from  below  {groiuid  moraiiie)  ;  the  one 
consists  of  loose  earth  and  angular  fragments,  the  other 
of  compact  clay  and  rounded,  striated  fragments.  Both 
are  wholly  unstratified. 

Evidences  of  Former  Extension  of  Glaciers. — 
The  characteristic  signs  of  glaciers,  therefore,  are  smooth, 
scored,  mow^o/j?;eef/ surfaces  of  rocks,  large  angular  perched 


AQUEOUS  AOENCIES. 


63 


blocks,  and  confused  piles  of  rubbish,  forming  terminal 
moraines.  It  is  by  evidence  of  this  kind  that  it  has  been 
proved  that  at  one  time  glaciers  were  far  more  extended 
than  now  in  regions  where  they  still  exist,  and  existed  in 
great  power  in  regions  where  they  are  no  longer  found  ; 
in  other  words,  that  there  was  once  a  glacial  epoch.  In 
the  Alps,  for  example,  polished,  scored,  moutonneed  rocks 
and  perched  bowlders  are  found  on  the  sides  of  the  valleys 
far  above  the  present  level  of  the  glacier  (old  glacial  flood- 
marks),   (Fig.   31)  ;  also,  smoothed,   scored,  moutonneed 


Fig.  31.— Ideal  section  of  a  glacier  bed.    a,  a,  present  level ;  6,  6,  and  c,  c, 
former  levels. 


rock-beds  and  deserted  terminal  moraines  are  found  far 
beyond  the  present  foot  of  the  glacier.  They  mark  the 
former  position  of  the  foot.  Behind  these  old  terminal 
moraines,  the  waters  flowing  from  the  glaciers  are 
dammed,  and  thus  form  lakes.  Similar  phenomena  are 
observed  in  regions  where  glaciers  no  longer  exist — as,  for 
example,  in  the  Sierra  and  Colorado  mountains.  As 
these  phenomena  belong  to  the  glacial  epoch,  it  is  best  to 
describe  them  in  connection  with  that  subject  (p.  386). 

Pot-holes  in  Old  Glacial  Beds. —  Pot-holes  of  enor- 
mous size  like  great  stone  wells,  10-15  ft.  wide  and  20-30 
ft.  deep,  are  often  found  in  old  glacial  beds.  They 
naturally  excite  the  wonder  of  the  inhabitants  of  Alpine 
regions  and  are  called  by  them  ''  Marmites  des  Geants,'* 
or  Mortars  of  the  Giants.  They  are  formed  not  by  the  ice 
but  by  the  rushing  waters  of  the  subglacial  streams,  or 


64 


DYNAMICAL  OEOLOOY, 


else  by  the  falling  of  superglacial  streams  through  great 
fissures  in  the  ice  (crevasses)  to  the  rocky  bed  below. 

Icebergs, 

We  have  already  stated  that  in  high-latitude  regions 
the  lower  limits  of  glaciers  touch  the  sea.  In  South 
America  this  occurs  in  45°  south  latitude  ;  in  Norway,  in 
65°  north  latitude  ;  and  in  Alaska,  in  60°  north  latitude. 
Still  nearer  the  poles  they  run  far  into  the  sea,  and  by 
the  buoyant  power  of  water,  and  the  up-and-down  move- 
ment of  tides  and  waves,  are  broken  off  in  great  prismatic 
masses,  and  float  away  as  icebergs.  In  the  North  Atlan- 
tic the  great  source  of  icebergs  is  Greenland  ;  in  the 
Southern  Ocean,  the  Antarctic  Continent. 

Greenland. — We  have  already  said  that  Greenland  is 
completely  covered  with  an  ice-mantle  2,000  to  3,000  feet 
thick,  which  moves  bodily,  by  slow  glacial  motion,  sea- 
ward, producing  doubtless  universal  erosion  ;  and  divides 
only  at  the  margin  into  separate  glaciers,  which,  running 
through  great  fiords,  thirty  to  forty  miles  long,  into  the 
sea,  there  form  icebergs.  These  are  then  taken  by  oceanic 
currents  and  carried  southward  into  warmer  waters,  where 
they  melt  (Fig.  32). 


Fig.  32.— Ideal  section  of  a  fiord  and  glacier,  forming  icebergs.    /,  I,  sea  level ;  6^, 
glacier  ;  i,  i,  i,  icebergs  ;  cl,  cliffs. 


It  is  easy  to  see  the  necessity  of  this  process.      The 
amount  of  snow  which  falls  in  polar  regions  is  far  greater 


AQUEOUS  AGENCIES, 


65 


than  the  waste  by  melting  and  evaporation  in  the  same 
regions.  If  there  were  no  means  of  disposing  of  the  ex- 
cess, it  would  accumulate  without  limit.  This  is  pre- 
vented, and  the  equilibrium  restored  by  the  running  off 
of  this  excess  into  the  sea  as  glaciers,  and  the  breaking 
off  as  icebergs,  which  then  float  away  to  warmer  latitudes, 
and,  there  melting,  are  returned  into  the  general  circula- 
tion of  meteoric  waters. 

Description. — The  coast  of  Greenland,  like  that  of 
Norway,  consists  of  bold,  rocky  headlands  and  deep  fiords 
and  high  islands  off  shore.  Into  each  fiord  runs  a  glacier, 
and//"o;?i  each  emerge  numberless  icebergs.  Baffin^s  Bay 
is,  therefore,  full  of  icebergs  of  all  sizes,  from  a  few  hun- 
dred feet  to  many  thousand  feet  on  a  side.  Sometimes 
several  hundred  may  be  seen  at  one  view.  They  are 
often  two  hundred  to  three  hundred  feet  high,  and,  since 
only  one  seventh  is  above  water,  some  of  them  must  be 


Pig.  33. 


at  least  two  thousand  feet  thick.  In  shape  they  are  at 
first  more  or  less  prismatic  (Fig.  32),  but  by  the  utiequal 
melting  of  the  sun  and  air,  they  become  finally  extremely 


Le  Conte,  Geol.  5 


66 


DYNAMICAL  GEOLOGY. 


irregular,  assuming  often  striking  forms  of  mountains, 
castles,  cathedrals,  etc.  (Fig.  33). 

In  Antarctic  regions  the  development  of  ice  is  even 
greater.  Whatever  of  land  exists  there,  is  completely 
covered  with  a  universal  ice-mantle,  which  pushes  out 
ten  miles  to  sea  as  a  continuous  ice-hariner.  From  the 
margin  of  this  barrier  break  off  regular  prismatic  blocks 
of  enormous  size  and  thickness  (Fig.  34) 


Pig.  34. 


Icebergs  as  a  Geological  Agent. 

Erosion. — Since  icebergs  are  floating  bodies,  they  do 
not  erode  unless  they  ground.  In  places  like  the  Banks 
of  Newfoundland,  where  icebergs  ground  in  great  num- 
bers, they  doubtless  disturb  the  bottom.  But,  when  we 
remember  the  irregularity  of  their  movements,  under  the 
action  of  waves  and  tides,  and  also  that  the  sea-bottom  is 
deeply  covered  with  ooze,  we  may  safely  assert  that  no 
smooth,  striated,  moutonneed  rocks,  such  as  are  produced 
by  glaciers,  would  be  found  there. 

Transportation  and  Deposit. — But,  in  the  trans- 
portation and  distribution  of  materials  very  widely  over 
the  sea-bottom,  the  agency  of  icebergs  is  very  important. 
The  Greenland  ice-sheet,  since  it  is  universal,  can  carry 
710  moraines  atop,  but  it  everywhere  pushes  seaward  its 


AQUEOUS  AGENCIES.  67 

firound  moraine.  In  addition  to  this,  as  soon  as  it  divides 
at  the  land-margin  into  separate  glaciers  which  run  down 
into  the  fiords,  each  separate  glacier  receives  its  burden 
of  material  from  the  cliffs  on  each  side  of  the  fiord,  and 
thus  becomes  loaded  with  earth  and  stones.  The  ice- 
bergs, therefore,  carry  away  immense  quantities,  both 
lying  on  their  surface  and  frozen  in  their  lower  parts. 
These  are  dropped  as  they  melt.  Thus,  the  land  of 
Greenland  is  being  cut  down  by  glacierp  and  carried  away 
by  icebergs,  and  strewed  all  over  the  bottom  of  the  At- 
lantic as  far  south  as  50°  to  40°  north  latitude.  The 
materials  thus  dropped  over  the  sea-bottom  are  similar 
to  those  borne  by  glaciers,  and  dropped  in  their  pathway, 
except  that  they  are  carried  much  farther,  and  also  that 
they  are  more  or  less  sorted  and  stratified  by  dropping 
through  water.  Large  blocks  perched  in  insecure  posi- 
tions would  not  probably  occur. 

Therefore,  smooth,  striated,  moutonnhd  rocks,  perched, 
angular  blocks,  and  terminal  moraines  cannot  be  produced 
by  icebergs,  but  are  wholly  characteristic  of  glaciers. 
The  importance  of  this  discussion  will  be  seen  "when  we 
come  to  speak  of  the  Glacial  epoch. 

Section  IV. — Chemical  Agency  of  Water. 

It  will  be  remembered  that  we  divided  the  agency  of 
water  into  mechanical  and  chemical.  We  have  now  fin- 
ished the  former,  and  are  ready  to  take  up  the  latter. 
Previous  to  doing  so,  however,  there  is  a  preliminary  sub- 
ject of  great  interest,  viz.,  underground  tuaters.  And 
here  we  return  again  to  the  domain  of  familiar  observa- 
tion, for  every  student  may  observe  for  himself  much  that 
follows : 

Underground  Waters  and  Origin  of  Springs, 

As  already  said  (page  18),  of  the  water  which  falls  by 
rain  and  snow,  a  part  runs  off  the  surface,  producing  uni' 


68  DYNAMICAL  GEOLOGY. 

versal  rain-erosion  ;  another  part  sinks  into  the  earth,  and 
after  a  longer  or  shorter  subterranean  course,  comes  up 
again  as  springs,  and  joins  the  surface-waters  to  form  the 
streams.  Still  a  third  part  does  not  come  up  on  the  land- 
surface  at  all,  but  by  subterranean  passages  finds  its  way 
to  the  sea.  In  some  countries  this  third  part  is  large. 
This  is  especially  so  if  the  country-rock  be  limestone  or 
lava,  for  these  rocks  are  affected  with  subterranean  gal- 
leries and  caves.  In  Florida,  for  example,  rivers  often 
disappear,  ingulfed  in  the  earth,  and  continue  to  the  sea 
by  subterranean  passages.  In  shallow  seas  olf  such  coasts 
places  are  known  where  fresh  water  comes  up  in  large 
quantities,  and  ships  may  be  supplied  with  drinking-water. 
Such  submarine  springs  are  known  off  the  coast  of  Florida, 
the  West  Indies,  the  Hawaiian  Isles,  and  the  shores  of 
the  Mediterranean.  There  is  still  another  part  of  sub- 
terranean water  which,  perhaps,  is  not  rain-water,  or,  if 
so,  is  not  noiu  circulating  like  the  others.  As  far  down 
as  the  earth  has  been  penetrated,  perhaps  below  even  the 
sea-bed,  there  is  found  rock-water,  but  not  in  flowing 
streams.  This  may  be  rightly  called  volcanic  water,  as  it 
is  probably  concerned  in  volcanic  eruptions. 

Perpetual  Ground-Water. — In  passing  downward 
from  the  surface  we  find  the  ground  contains  more  and 
more  water,  until  it  becomes  saturated  and  the  water  mov- 
able. The  highest  level  at  which  the  Avater  is  ahuays 
movable  is  called  the  level  of  perpetual  ground-water.     It 


Fig.  35.— Diagram  of  perpetual  ground-water.    S,  surface  of  ground  ;  Tf  TV,  ground- 
water ;  sp,  sp,  hillside  springs. 


is  deepest  on  che   iiilltops,  comes  nearer  the  surface  on 
the  slopes,  and  under  favorable  conditions  may  reach  the 


AQUEOUS  AGENCIES.  69 

surface  near  the  bottom  or  in  the  valleys  and  issue  as 
springs  (Fig.  35). 

Springs. — Springs  are  the  issuance  of  underground 
waters,  and  wells  are  artificial  springs.  Often  water  is 
observed  to  ooze  out  on  hillsides  or  at  hill-bottoms,  mak- 
ing a  marshy  spot.  In  such  cases,  if  we  examine,  we  shall 
usually  find  a  reason  for  it  in  the  fact  that  a  water-bear- 
ing stratum  of  sand  or  gravel,  underlaid  by  an  impervious 
stratum  of  clay,  outcrops  at  this  point.  Water  falling  on 
the  hill  sinks  down  until  it  reaches  the  impervious  clay, 
and  then  flows  out  laterally  (Fig.  36).  These  may  be 
called  seepage-springs. 

Again  :  in  other  places,  especially  mountain-regions, 
we  find  strong  or  bold  springs.     Usually,  in  such  cases,  we 


Fia.  36.— Hillside  spring 


spring. 


may  find  a  fissure  through  which  the  water  comes  up  to 
the  surface  (Fig.  37). 

In  still  other  places,  but  only  in  countries  where  rocks 
of  cavernous  structure,  such  as  limestone  and  lava,  prevail. 


Fig.  37.— Fissure  spring. 

we  sometimes  find  great  springs,  from  which  issue  rivers 
of  considerable  size.  Perhaps  the  most  remarkable  ex- 
ample is  the  '^^  Silver  Spring"  of  Florida.  The  river 
which  flows  from  this  celebrated  spring  is  so  considerable 


70  DYNAMICAL   GEOLOGY. 

that  small  steamers  go  up  the  river  and  i7ito  the  spring^ 
and  the  cotton  of  the  region  is  shipped  at  the  Silver 
Spring  landing.  For  fifty  to  sixty  miles  around,  there 
are  no  surface-streams.  The  country-rock  being  here  a 
very  soft  and  cavernous  limestone,  the  rain-water  is  all 
absorbed  and  finds  its  way  by  underground  streams  to  the 
surface  at  Silver  Spring.  This  spring  is  also  celebrated 
as  having  probably  the  clearest  water  in  the  world. 

Artesian  Wells. — Ordinary  wells  are  artificial  seepage- 
springs.  Artesian  wells  are  artifical  great  spriiigs,  i.  e., 
they  are  the  tappings  of  underground  streams  which  other- 
wise would  have  reached  the  sea  without  coming  to  the 
surface.  In  a  level  country,  underlaid  by  regular  strata 
which  turn  up  and  outcrop  on  mountains  or  hills  (Fig. 
38),  we  are  almost  certain  to  reach  artesian  water.     In 


Fig.  38.— Artesian  well. 

such  cases,  the  pressure  of  water  from  the  hills  will  cause 
the  water  to  rise,  not  only  to  the  surface,  but  often  to 
spout  as  a  fountain.  Fig.  38  shows  the  conditions  under 
which  this  will  probably  occur.  There  are  few  places 
where  artesian  water  may  not  be  reached  by  deep  boring, 
though  the  conditions  of  abundant  supply  are  by  no  means 
universal.  The  deepest  artesian  wells  are  those  near 
Leipzig,  5,735  feet  deep  ;  near  Pittsburg,  Pa.,  4,625  feet : 
at  St.  Louis,  nearly  4,000  feet.  Water  from  such  greai 
depths  is  aWays  warm. 

Thus,  then,  rain-water  falling  upon  the  land  returns  to 
sea,  whence  it  came,  partly  by  surface  drainage,  partly  h^ 


AQUEOUS  AGENCIES.  71 

underground  drainage.  The  relative  proportion  of  these 
varies  greatly  in  different  countries,  depending  upon  the 
nature  and  position  of  the  rocks ;  but  by  far  the  larger 
part  usually  returns  by  the  surface,  being  brought  up  by 
hydrostatic  pressure.  Only  in  limestone  and  in  recent 
volcanic  regions  is  the  proportion  returning  by  under- 
ground passages  large. 

Chemical  Effects  of  Underground  Waters  ;  Mm-, 
eral  Springs. — We  have  seen  that  all  rocks  are  changed 
into  soils  by  the  removal  of  their  soluble  portions.  These 
are  then  taken  up  by  percolating  water  and  brought  to  the 
surface  by  springs,  while  the  insoluble  portions  remain  as 
soils.  It  is  evident,  then,  that  all  springs  contain  mineral 
matters  derived  from  the  rocks.  If  the  quantity  be  large, 
or  the  mineral  rare  and  medicinal,  then  it  is  called  a  min- 
eral spring.  These  are  usually  hot  because  of  the  great 
solvent  property  of  hot  water. 

Ljimestone  Caves. — Now,  in  most  cases  the  propor- 
tion of  insoluble  matter  is  so  large  that  the  resulting  soil 
is  fully  as  bulky  as  the  rock  from  which  it  was  formed  ; 
there  is,  therefore,  no  vacant  space  left.  But,  in  the  case 
of  limestone,  the  whole  rock  is  soluble  in  water  containing 
CO3.  Underground  streams,  therefore,  dissolve  out  gal- 
leries and  caves  in  their  courses.  Hence,  irregular  caves 
and  galleries  are  found  in  limestone  rocks  in  all  countries. 
These  are  often  of  very  great  extent,  the  galleries  being 
sometimes,  as  in  the  case  of  the  Mammoth  Cave,  hun- 
dreds of  miles  long.  The  most  celebrated  in  this  country 
5,re  the  Mammoth  Cave,  Kentucky  ;  Wyandotte  Cave, 
Indiana ;  Wier  and  Luray  Caves,  Virginia ;  Nicojack 
Cave,  Tennessee  ;  and  Bower  Cave,  in  California. 

In  all  cases  they  have  been  hollowed  out  by  solution  and 
by  erosion,  and  therefore  are,  or  have  been,  occupied  by 
underground  streams.  Some  are  so  still,  as  the  Nicojack  ; 
5ome  only  partly,  as  the  Mammoth  Cave  ;  some  not  at  all. 
In  all  cases,  they  have  been  occupied  in  former  times  by 


72 


DYNAMICAL  GEOLOGY. 


much  larger  streams  than  now ;  in  nearly  all  cases, 
instead  of  being  hollowed  out  by  solution,  they  are  now 
being  filled  again  by  chemical  deposit.  When  the  waters 
were  in  abundance  they  dissolved  ;  but,  now  that  they  are 
reduced  to  drippings,  they  deposit.  The  drippings  from 
the  roof  form  icicle-like  stalactites  (a)  ;  the  drippings  on 
the  floor,  the  stalagmites  {b)  ;  the  runnings  down  the 
walls  form  pilasters.  The  stalactites,  constantly  growing 
in  size  and  length,  finally  meet  the  stalagmites  and  form 
pillars  (c),  (Fig.  39). 


Section  of  limestone  cave. 


The  variety  and  beauty  of  the  forms  produced  by 
deposit  in  these  caves  are  often  marvelous.  One  of  the 
latest  discovered  and  most  wonderful  in  this  respect  is  the 
Luray  Cave,  Virginia,  described  in  the  '^  Smithsonian 
Keport '"  for  1880.     Fig.  40  is  copied  from  this  report. 

The  caves  and  galleries  of  lava  are  formed  in  an  entirely 
different  way,  as  will  be  explained  hereafter  under  the 
head  of  Igneous  Agencies  (page  137). 

Lime-Sinks. — In  cases  of  soft,  impure  limestone,  the 
undermining  of  the  rock  causes  the  surface  to  sink. 
Thus  are  formed  the  lime  sinks  so  common  in  Florida 
and  some  portions  of  Georgia. 

Chemical  I>eposits  in  Spring's. — We  have  seen  that 
all  springs  contain  mineral  matters  in  solution.  Some  of 
these  are  de])osited  at  the  surface,  and  some  not.  The 
most  important  deposits  are  lime  carbonate  from  carbon- 


AQUEOUS  AGENCIES. 


73 


ated  springs,  iron  oxides  from  chalybeate  springs,  sulphur 
from  sulphur-springs,  and  silica  from  alkaline  springs. 


YiQ.  40.— The  Saracens'  tent,  Luray  Cave. 


Deposits  of  Lime  Carbonates. — These  deposits  are  in 
carbonated  springs  in  limestone  regions,  and  especially 
in  volcanic  countries.  In  order  to  understand  why 
deposit  occurs,  the  student  must  remember — 1.  That 
lime  carbonate  (limestone)  is  slightly  soluble  in  water 
containing  CO,.  2.  That  up  to  a  certain  limit  the  solu- 
bility is  proportioned  to  the  amount  of  CO,.  3.  That  the 
amount  of  CO,  taken  up  by  water  is  proportioned  to  the 
pressure.  Now,  besides  the  small  quantity  of  CO,  in  air, 
and  therefore  in  all  meteoric  water,  there  are  also  subter- 
ranean sources,  especially  in  volcanic  regions.  Therefore, 
if  water  circulating  deep  in  the  interior,  and  therefore 
under  heavy  pressure,  come  in  contact  with  subterranean 
sources  of  CO,,  it  will  take  up  a  corresponding  amount. 


74  DYNAMICAL  GEOLOGY, 

and  coming  up  to  the  surface,  and  the  pressure  being 
relieved,  a  portion  of  the  00,  will  escape,  with  efferves- 
cence. Thus  are  formed  carbonated  springs,  often  called 
soda  spring Sy  on  account  of  their  pleasant  pungency,  like 
the  so-called  soda  water  of  the  shops.  Now,  if  such  water, 
thus  charged  with  CO^,  meets  limestone,  it  will  dissolve 
a  proportionate  amount  of  this  substance.  On  coming 
to  the  surface,  the  pressure  being  relieved  and  the  CO, 
escaping,  the  lime  carbonate  will  be  deposited  about  the 
spring  and  in  the  course  of  the  issuing  stream  as  long  as 
the  CO,  continues  to  escape. 

In  this  way  immense  deposits,  several  hundred  feet 
thick  and  many  miles  in  extent,  are  formed.  If  the 
deposits  are  regular  and  slow,  the  rock  formed  will  be 
hard  {travertine),  but,  if  rapid  and  with  escaping  gas,  it 
is  spongy  {calcareous  tufa).  If  the  water  be  free  from 
coloring-matter,  the  stone  is  exquisitely  white  and  fine  ; 
but  if  iron  be  present,  it  will  be  yellowish,  buff,  or 
brown.  If  the  coloring-matter  varies  from  time  to  time, 
the  most  exquisitely  banded,  striped,  and  clouded  appear- 
ance results.  Nothing  can  exceed  the  delicate  beauty  of 
these  deposits  in  some  cases.  If  the  water,  thus  highly 
charged  with  lime  carbonate,  makes  a  cascade,  ever}; 
object  on  which  the  spray  falls  becomes  covered  with 
deposit.  In  Italy,  advantage  is  taken  of  this  property  to 
make  facsimiles  of  coins,  medallions,  etc.  The  stream 
from  the  spring  is  made  to  fall  on  lattice,  which  scatters 
the  spray  in  all  directions.  The  medallions  are  placed 
on  shelves  within  reach  of  the  spray,  and  quickly  be- 
come incrusted.  The  removed  crust,  similarly  placed, 
is  used  as  a  mold,  in  which,  by  deposit,  a  facsimile  is 
made. 

Examples  of  such  deposits  are  found  in  all  parts  of  the 
world.  Perhaps  the  most  beautiful  occur  in  Italy,  where 
exquisite  works  of  art  are  made  of  them.  But  many 
examples  are  found  in  our  own  country.     About  the  Old 


AQUEOUS  AGENCIES. 


.     76 


Sweet  and  the  Eed  Sweet  Springs,  West  Virginia,  and  in 
the  course  of  the  issuing  stream,  a  buff-colored  travertine 
is  deposited.  About  two  miles  below  the  Red  Sweet  the 
stream  makes  the  Beaver-dam  Falls.  Here  everything 
within  reach  of  the  spray— leaves,  twigs,  grass — becomes 
quickly  coated  with  deposit.  In  California,  the  exqui- 
sitely banded  Suisun  marble  is  a  deposit  formed  in  earlier 
geological  times.  In  Yelloivstone  Parle,  the  deposits  of 
this  kind  are  very  abundant,  and  assume  strange  and 
beautiful  forms  (Fig.  41). 


Fig.  41.— Deposits  of  lime  carbonate,  Yellowstone  Park.    (After  Hayden.) 


Deposits  of  Iron  Oxide. — Iron  is  one  of  the  most 
universally  diffused  of  substances.  In  the  form  of  car- 
bonate and  sulphate,  especially  the  former,  it  is  dis- 
solved in  the  water  of  chalybeate  springs.  Every  one 
must  have  observed  that  about  such  springs  there  is  a 


76  DYNAMICAL  GEOLOGY. 

reddisn  deposit  ;  in  fact,  such  deposit  is  the  usual  sign  of 
the  existence  of  iron  in  the  waters. 

The  explanation  of  the  deposit  is  as  follows  :  Iron  has 
a  powerful  affinity  for  oxygen.  As  soon,  therefore,  as  the 
water  reaches  the  surface  the  iron  exchanges  00,  for  oxy- 
gen of  the  air  and  forms  a  peroxide,  which,  being  insol- 
uble, is  deposited.  In  regard  to  how  the  iron  came  in 
the  solution,  we  shall  speak  again  under  Organic  Agency. 

Deposits  of  Sulphur — A  yellowish  deposit  is  usually 
seen  about  sulphur-springs.  These  springs  contain  hydric 
sulphide  (H^S)  in  solution.  The  oxidation  of  this  by 
the  contact  of  air  forms  water  and  deposits  sulphur  (H^S 
+  0  =  H,0  +  S). 

Deposits  of  Silica. — Silica  (quartz,  sand,  flint,  etc.)  is 
usually  regarded  as  extremely  insoluble,  but  it  is  soluble 
to  a  limited  extent  in  alkaline  carbonate  waters,  and  the 
solubility  increases  with  the  heat.  Now,  alkaline  carbon- 
ate waters  are  common  in  volcanic  regions,  and  are  often 
hot.  Such  hot  alkaline  springs  take  up  silica  in  their 
subterranean  course,  and,  coming  to  the  surface,  deposit 
abundantly,  partly  by  cooling,  mostly  by  drying.  Perhaps 
the  best  example  of  such  deposits  is  found  at  Steamboat 
Springs,  Nevada.  Here,  over  an  area  of  half  a  mile  long 
and  a  quarter  of  a  mile  wide,  the  whole  surface  is  covered 
with  a  deposit  of  silica  twenty  feet  thick,  and  over  the 
whole  area  clouds  of  steam  are  seen  issuing  from  many 
vents.  The  deposit  takes  a  great  variety  of  forms — some- 
times a  tufaceous  material  called  sinter  ;  sometimes  more 
solid  and  regularly  banded  ;  sometimes  milky-white  chal- 
cedony ;  and  sometimes  white  quartz  like  loaf  sugar.  De- 
posits of  silica  are  found  in  all  geysers.  We  shall,  there- 
fore, again  speak  of  this  under  that  head. 

Chemical  Deposits  in  Lakes. 

Saline  Lakes ;  Salt  Lakes.— Salt  lakes  are  found 
only  in   dry  climates.     They  are  formed  in  two  ways — 


AQUEOUS  AGENCIES,  77 

either,  a,  by  indefinite  concentration  of  river  water  in  a 
lake  without  outlet ;  or,  h,  by  isolation  of  a  portion  of 
sea  water  by  movement  of  the  earth's  crust  (upheaval  of 
sea  bed).  The  salt  lakes  scattered  over  the  Nevada 
Basin — e.  g.,  Pyramid,  Winnemucca,  Carson,  Humboldt, 
and  Walker  Lakes,  etc. — probably  belong  to  the  former 
class,  for  some  of  them  are  but  slightly  salt  even  yet. 
Great  Salt  Lake  has  been,  regarded  as  belonging  to  the 
latter  class,  but  if  it  once  had  an  outlet,  as  now  seems 
certain,  it  must  have  been  formed  like  all  the  other  lakes 
\n  this  region.  The  Dead  Sea,  from  the  composition  of  its 
Jvater,  is  regarded  as  an  example  under  the  second  class. 
The  Caspian  is  usually  regarded  as  an  example  under  the 
first  class,  for,  though  it  has  apparently  dried  away  from 
much  greater  dimensions,  yet  its  waters  are  much  less 
salt  than  those  of  the  ocean.  Nevertheless,  there  are 
some  reasons  for  thinking  that  the  Caspian  was  once  con- 
nected with  the  Black  Sea  and  with  the  Arctic  Ocean. 

Alkaline  Lakes. — Saline  lakes  are  of  two  principal 
kinds,  salt  and  alkaline.  All  those  mentioned  above  are 
salt.  Alkaline  lakes  are  rare  ;  they  are  found  in  Hungary, 
Lower  Egypt,  and  especially  in  Nevada  and  California. 
The  largest  of  these  are  Lakes  Mono  and  Owen,  both  in 
California.  The  waters  of  Lake  Mono  are  a  strong  solu- 
tion of  sodic  carbonate  (sal-soda),  with  some  carbonate  of 
lime,  borax,  and  salt.  Lake  Owen  contains  about  equal 
I  parts  of  soda  and  salt.  Salt  lakes  may  be  formed  in  either 
of  two  ways,  but  alkaline  lakes  in  only  one — viz.,  by  in- 
definite concentration  of  river  or  spring  water  in  a  lake 
without  an  outlet.  If  in  the  river  or  spring  water  alkaline 
chlorides  predominate,  the  lake  will  be  salt ;  if  alkaline 
carhonates  predominate,  it  will  be  alkaline. 

Borax  Lakes. — These  are  still  rarer  than  alkaline 
lakes.  They  are  found  only  in  Thibet  and  in  California 
and  Nevada.  Borax  lakes  are  of  course  formed  only  by 
concentration  of  spring  water. 


78  DYNAMICAL  OEOLOOY. 

Conditions  of  the  Formation  of  Saline  Lakes. — 

Any  lake  will,  in  time,  become  saline  if  it  has  no  outlet  ; 
but  whether  or  not  it  has  an  outlet  depends  on  the  relation 
of  supply  by  rivers  and  springs  to  waste  by  evaporation. 
If  the  supply  exceeds  the  waste,  the  lake  will  rise  until  it 
finds  an  outlet,  and  remain  fresh — i.  e.,  the  quantity  of 
saline  matter  is  so  small  as  to  be  imperceptible  to  the 
taste.  But  if  the  waste  be  equal  to  or  exceed  the  supply, 
so  that  the  quantity  of  water  is  stationary  or  diminishes, 
then  the  salting  process  begins.  Every  drop  of  river  or 
spring  water  coming  into  the  lake  contains  some  saline 
matter,  however  small,  gathered  from  rocks  and  soils, 
and  this  is,  of  course,  left  in  the  lake.  Thus  there  is  a 
continual  leaching  of  all  the  surrounding  soils,  and  an 
accumulation  of  the  leachings  in  the  lake.  It  may  take 
thousands  or  even  hundreds  of  thousands  of  years,  but  in 
time  the  lake  will  become  saline,  and  more  and  more  so, 
until  finally  the  point  of  saturation  is  reached  and  deposit 
begins. 

I  have  taken  the  case  of  concentration  of  river  water, 
but  it  matters  not  how  the  lake  was  originally  formed  ; 
the  same  conditions  will  determine  its  saltness  or  fresh- 
ness. For  example,  if  a  salt  lake  be  formed  by  the  isola- 
tion of  a  portion  of  sea  water  in  the  gradual  upheaval  of 
a  continent,  whether  it  remains  a  salt  lake  or  whether  it 
becomes  fresh  will  depend  on  the  conditions  indicated 
above.  If,  for  example,  the  sea  bottom  and  contiguous 
land  about  the  Golden  Gate  were  elevated  so  as  to  sepa- 
rate the  Bay  of  San  Francisco  from  the  Pacific  Ocean, 
this  bay  would  certainly  become  a  fresh  lake  ;  for  the 
amount  of  water  coming  into  the  bay  is  far  greater  than 
the  waste  by  evaporation.  This  is  shown  by  the  fact 
that,  although  so  fully  connected  with  the  ocean,  the 
waters  of  the  bay  are  far  fresher  than  those  of  the  sea. 
The  water  of  the  bay  would  therefore  rise  until  it  found 
an  outlet  to  the  sea,  and  then  would  commence  a  process 


AQUEOUS  AGENCIES.  79 

of  rinsing  out  by  fresh  water,  until  it  would  become 
entirely  fresh.  For  the  same  reason,  it  is  believed  that 
the  Black  and  Baltic  Seas,  if  cut  off,  would  become  fresh, 
while  the  Gulf  of  California  and  the  Mediterranean  would 
not  only  remain  salt,  but  would  become  more  and  more 
salt  until  they  would  deposit.  Lake  Champlain,  as  we 
shall  see  hereafter  (p.  391),  was  once  connected  with  the 
Atlantic.  When  first  separated,  it  was  of  course  salt,  but 
by  the  continual  pouring  in  of  fresh  water  and  pouring 
out  of  the  mixture,  the  lake  was  gradually  rinsed  out  and 
became  fresh. 

It  is  evident,  then,  that  we  ought  to  find  saline  lakes 
only  in  very  dry  climates ;  for,  in  most  places,  the  rain 
falling  on  land-surface  is  far  greater  than  the  evapo- 
ration from  the  same,  the  excess  finding  its  way  to  the 
sea  by  rivers.  It  is  evident,  also,  that  we  ought  to  find 
every  degree  of  salination  of  salt  lakes.  Lake  Walker, 
Nevada,  and  Lake  Tulare,  California,  are  but  slightly 
saline,  while  Great  Salt  Lake,  Utah,  is  already  saturated 
and  beginning  to  deposit ;  and  many  examples  of  dried-up 
salt  lakes  are  found  all  over  Utah  and  Nevada.  It  is 
evident,  again,  that  only  the  last  reservoir^  even  of  the 
same  river  water,  will  be  salt.  Thus  the  same  water  runs 
into  Lake  Tahoe,  and  thence  through  Truckee  River  into 
Pyramid  Lake — but  no  farther.  Lake  Tahoe  is  deli- 
ciously  fresh,  while  Pyramid  Lake  is  salt.  So,  also,  the 
same  water  runs  into  Lake  Utah,  and  thence,  through 
the  River  Jordan,  into  Great  Salt  Lake,  but  no  farther. 
Lake  Utah  is  fresh ;  Great  Salt  Lake  is  a  saturated 
brinCo 

Why  the  Ocean  is  Salt. — The  ocean  is  a  reservoir 
without  outlet.  But  the  question  of  its  saltness  is  a  little 
different  from  that  of  salt  lakes ;  for  much  of  the  rocks 
of  the  earth^s  crust  was  formed  at  the  sea  bottom  and 
salted  by  the  sea  ;  and  the  salt  is  only  regathered  in  salt 
lakes.     Nevertheless  there  can  be  no  doubt  that  the  sea 


so  DYNAMICAL   GEOLOGY. 

also  got  its  salt  from  the  rocks.  The  salts  of  the  sea 
are  the  accumulated  leachings  of  all  Geological  times. 

Deposits  in  Saline  Lakes. — We  have  seen  that  saline 
lakes  occur  only  in  dry  climates.  Furthermore,  the  cli- 
mate of  the  regions  where  they  occur  has  been  for  a  long 
time,  and  still  is,  groiving  drier.  The  lakes  have  been, 
and  are  still,  drying  up,  and  many  have  entirely  dried 
away.  In  the  Basin  region — i.  e.,  the  desert  region  be- 
tween the  AYahsatch  and  Sierra  ranges — there  are  hun- 
dreds of  such  dried-up  lakes.  Now,  from  the  time  the 
point  of  saturation  is  reached  until  the  lake  is  dried  up, 
deposits  of  some  kind  must  occur.  These,  of  course,  vary 
with  the  composition  of  the  water,  but  the  simplest  are 
those  formed  by  the  drying  up  of  an  isolated  body  of  sea 
water.  What  will  take  place  in  such  a  case  is  known  by 
the  artificial  evaporation  of  sea  water  in  the  manufacture 
of  salt.  No  deposit  at  all  takes  place  until  nine  tenths  of 
the  water  is  evaporated.  Then,  as  the  point  of  saturation 
for  salt  is  approached,  first  gypsum  is  deposited  ;  after  the 
whole  of  the  gypsum  is  deposited,  the  common  salt  begins 
to  deposit,  and  continues  until  nearly  all  is  crystallized, 
and  a  dense  mother  liquor,  or  bittern,  is  left,  containing 
the  more  soluble  matters,  especially  magnesium  chloride 
and  sulphate. 

In  Nature  the  process  is  the  same,  except  that  it  is 
complicated  by  mechanical  deposits  of  silt  (sand  and 
clay).  Until  the  point  of  saturation  is  reached,  only 
mechanical  deposits  of  silt  take  place,  as  in  fresh  lakes. 
Then  gypsum  will  begin  to  deposit,  but  not  continuously. 
In  all  such  dry  countries,  all  the  rain  that  falls  at  all,  falls 
in  a  few  months  of  each  year.  The  deposit  of  gypsum 
will  alternate  witli  mechanical  deposits  of  mud  or  silt. 
The  chemical  deposits  of  gypsum  will  represent  the  dry 
season,  and  the  mechanical  deposits  of  silt  the  season  of 
rains.  When  all  the  gypsum  is  exhausted,  the  common 
salt  will  begin  to  deposit,  and  this  will  alternate  in  the 


AQUEOUS  AGENCIES.  81 

same  way  with  silt,  until  finally  only  a  mother  liquor  is 
left,  which  is  very  difficult  to  dry  away. 

Now,  all  these  stages  are  actually  found  in  Nature. 
Great  Salt  Lake  has  reached  the  saturation  point,  and  is 
beginning  to  deposit.  The  Dead  Sea  has  deposited  largely, 
and  its  composition  is  that  of  a  half  exhausted  mother 
liquor.  Lake  Elton,  on  the  Russian  steppes,  has  deposited 
all  its  salt,  and  its  composition  is  that  of  a  wholly  exhausted 
mother  liquor.  Borings  about  the  shores  of  Lake  Elton 
show  an  alternation  of  silt  and  salt  many  times  repeated, 
such  as  we  have  described.  The  bearing  of  these  facts 
on  the  explanation  of  what  are  called  salt-measures  will 
be  seen  when  we  come  to  treat  of  these. 

In  cases  of  saline  lakes  formed  by  the  accumulation  of 
the  waters  of  rivers  and  springs,  the  deposits  are  far  more 
complicated.     The  deposits  found  in  the  dried-up  lakes 


Fig.  42.— Au  island  of  tufa  in  Pyramid  Lake,  Nevada. 

of  Nevada  consist  of  common  salt,  lime  carbonate,  soda 
sulphate,  soda  carbonate,  soda  borate  (borax),  and  soda 
lime  borate  (ulexite). 

Lk  Contk,  Geol.  6 


82  DYNAMICAL-  GEOLOGY. 

In  alkaline  lakes,  like  Mono,  immense  quantities  oi 
lime  carbonate  are  depositing  now,  apparently  from  hot 
springs  containing  lime,  coming  up  in  the  bed  of  the  lake. 
These  deposits  take  on  curious  coralline  forms,  which  are 
very  characteristic.  Similar  rough  coralline  forms  are 
seen  all  about  the  margin  and  in  the  shallow  water  of  this 
lake,  looking  at  a  distance  like  the  dead  stumps  of  an  old 
forest.  They  show  a  greater  extent  of  the  lake  at  one 
time  than  now.  Immense  deposits  of  a  similar  kind  are 
found  in  many  parts  of  Nevada,  and  mark  the  places  of 
dried-away  lakes  (Fig.  42). 


CHAPTER  HI. 
orga:n^ic  agencies. 

Organic  agencies  are  less  powerful  than  aqueous  in 
modifying  the  surface  of  the  earth  ;  yet,  even  in  this 
respect,  they  are  of  no  mean  importance,  since  enormous 
beds  of  limestone  are  formed  by  this  means.  But  their 
true  importance  is  perceived  when  we  remember  that 
organisms  are  the  most  delicate  indicators  of  physical 
conditions,  and  therefore  of  the  changes  through  which 
the  earth  has  passed.  Organic  remains  or  fossils  are,  as 
it  were,  the  characters  in  which  the  history  of  the  earth 
is  written. 

The  subject  may  be  best  treated  under  four  heads,  each 
having  a  special  application  in  explaining  some  important 
point  in  the  history  of  the  earth,  viz. :  1.  Vegetable  accumu- 
lations,  to  throw  light  on  the  formation  of  coal  and  lignite. 

2.  Iron  accumulations f  to  throw  light  on  the  great  beds  of 
iron-ore,  found  in  the  strata  of  earlier  geological  times. 

3.  Lime  accumulations,  to  explain  the  formation  of  lime- 
stones. 4.  Geographical  distributio7i  of  species,  to  throw 
light  on  the  geographical  diversity  of  species  in  earlier 
epochs,  and  on  the  laws  of  succession  of  organic  forms 
in  the  history  of  the  earth — i.  e.,  the  laws  of  evolution. 

The  phenomena  under  all  these  heads  can  be  observed 
by  each  one  for  himself  ;  but  it  must  be  remembered  that 
nearly  all  geological  causes  are  very  slow  in  their  opera- 
tion, and,  therefore,  the  phenomena  are  not  forced  upon 
our  attention,  but  must  be  looked  for  by  intelligent,  ever- 
83 


84  DYNAMICAL   GEOLOGY. 

watchful  observation.  It  is  for  this  reason  that  geologi- 
cal phenomena  are  peculiarly  adapted  to  cultivate  the 
habit  of  observation. 

Section  I. — Vegetable  Accumulations. 
Peat-Bogs. 

Definition. — Every  one  knows  that,  under  certain 
conditions,  especially  a  moist  climate  and  imperfect  drain- 
age, and  in  certain  spots  where  moss,  rushes,  and  other 
water-loving  plants  grow,  there  is  found  a  black,  carbona- 
ceous mud,  often  many  feet  deep.  A  surface-crust  is 
formed  on  the  interlacing  roots  of  many  kinds  of  plants, 
beneath  which  is  a  tremulous  mass  of  semi-liquid  matter. 
On  the  surface-crust  men  or  animals  venturing,  sometimes 
break  through,  and  are  ingulfed  and  perish.  Such  car- 
bonaceous mud  is  called  peat,  and  the  places  where  it 
accumulates,  peat-m,osses  or  yeat-bogs. 

Peat-bogs  are  most  common  in  cool,  moist  climates. 
A  large  part  of  Ireland,  Scotland,  Norway,  Sweden,  and 
Northern  Europe  generally,  is  covered  with  them.  They 
cover,  also,  large  parts  of  New  England,  and  especially 
of  Canada.  In  California,  though  a  dry  climate,  an  im.- 
perfect  peat  is  found,  covering  large  areas  on  the  Lower 
Sacramento  and  the  San  Joaquin  Rivers.  These  are  the 
'Hule-lands."  In  tropic  and  semi-tropic  countries,  accu- 
mulations of  peat  are  not  so  common,  but  are  on  a  grander 
scale.  The  peaty  accumulations  there  are  overgrown,  not 
by  moss  and  rushes  and  shrubs,  but  by  great  swamp-trees. 
In  these  countries  we  have  not  so  many  peat-bogs,  but  a 
few  great  peat-swamps.  Examples  of  these  are  found  in 
the  Great  Dismal  Swamp  of  Virginia  and  North  Carolina, 
and  in  the  great  peat-swamps  of  the  river-swamp  and  delta 
of  the  Mississippi. 

Structure  and  Composition  of  Peat. — Beginning  at 
the  surface,  we  have  in  a  peat-bog  first  the  living  vegeta- 


ORGANIC  AGENCIES.  86 

tion  and  the  undecomposed  remains  of  the  recently  dead. 
As  we  pass  down,  the  remains  become  older,  and  there- 
fore more  and  more  decomposed,  and  darker  in  color, 
until,  at  sufficient  depth,  it  is  a  black  mud,  structureless 
to  the  naked  eye,  though  still  revealing  vegetable  struc- 
ture to  the  microscope.  In  composition  it  is  mainly  car- 
bon, with  variable  proportions  of  the  hydrogen,  oxygen, 
and  nitrogen  of  the  original  plants.  It  is  a  disintegrated 
vegetable  matter,  which  has  lost  much  of  its  gaseous 
elements,  and  therefore  with  an  excess  of  carbon.  It  is 
therefore  a  good  fuel,  and  is  extensively  cut  and  used  for 
this  purpose,  either  simply  dried  or,  better,  made  into  a 
cake  by  hydraulic  pressure. 

Antiseptic  Property, — Peat  has  a  remarkable  power 
of  preventing  or  retarding  decomposition.  Logs  and 
stumps  have  been  found  buried  fifteen  to  twenty  feet  in 
peat,  and  therefore  probably  hundreds  and  even  thousands 
of  years  old,  which  are  still  in  a  sound  condition,  and 
even  fit  for  timber.  Bodies  of  men  and  animals  have  been 
found  with  even  the  flesh  preserved,  though  changed  into 
adipocere.  According  to  Lyell,  the  body  of  a  man, 
clothed  in  coarse  hair-cloth,  was  found  in  an  Irish  bog  ; 
and  in  a  bog  in  Lincolnshire,  the  body  of  a  woman,  with 
skin,  nails,  and  hair  preserved,  and  with  sandals  on  the 
feet.  The  sheletons  of  men  and  animals  thus  preserved 
are  much  more  common  ;  and  even  the  skeletons  of  extinct 
species  have  been  found  in  a  perfect  condition  and  un- 
petrified.  The  finest  specimens  of  the  mastodon  have 
been  obtained  from  old  bogs  in  New  York,  New  Jersey, 
Ohio,  and  Missouri. 

Mode  of  Accumulation. — Remembering  the  antiseptic 
property  of  peat,  its  mode  of  accumulation  is  easily  under- 
stood. In  forests,  a  layer  of  mold  a  few  inches  thick  ac- 
cumulates on  the  soil  from  decomposition  of  the  annual 
leaf-fall.  This  will  not  thicken  indefinitely,  because  the 
rate  of  complete  decomposition  quickly  equals  the  rate  of 


86  DYNAMICAL  GEOLOGY, 

addition.  But,  if  abundant  water  be  present,  then  the 
peculiar  change  takes  place  by  which  peat  is  formed,  and 
the  antiseptic  property  of  the  peat  prevents  complete  de- 
composition, and  the  vegetable  matter  accumulates  with- 
out limit.  Thus,  a  peat-bog  represents  the  accumulated 
remains  of  thousands  of  generations  of  plants.  Every 
year  adds  to  the  ancestral  funeral -pile,  and  the  peat- 
ground  rises  higher  and  higher,  until,  although  commenc- 
ing on  a  low  spot,  it  may  rise  abo  ve  the  immediately  sur- 
rounding region,  and,  when  swollen  by  rains,  may  even 
burst  and  deluge  the  surrounding  country  with  black 
mud.  In  the  case  of  the  great  peat-swamps  of  southern 
regions,  the  accumulation  is  entirely  in  this  way — i.  e., 
by  growth  in  place.  But,  in  small  peat-bogs  in  hilly 
countries,  the  peat  accumulates  also  by  the  driftage  of 
surface-mold.  In  this  case,  the  accumulation  is  much 
more  rapid,  but  the  peat  is  less  pure. 

Lastly  :  As  a  peat-swamp  commenced  on  a  low  spot,  it 
was  often,  at  first,  a  shallow  pond  or  lake,  and  the  peaty 
matter  encroached  upon  it  from  the  margin.  Thus,  there 
may  be  found  in  the  center  of  the  peat- swamp  a  small 
remnant  of  the  original  lake.     The  Great  Dismal  Swamp 


Pig.  43.— Ideal  section  across  the  Great  Dismal  Swampo 

is  an  excellent  example.  This  swamp,  forty  by  thirty 
miles  in  extent,  is  overgrown  with  great  swamp-trees  sg 
thickly  that  there  is  little  or  no  underbrush.  The  peat 
accumulates  by  the  annual  fall  of  leaves  and  branches 
only,  and  the  rate  of  thickening  ie,  therefore,  probably 
very  slow.  The  peat  is  very  black,  pure,  and  structure- 
less, and  is  from  twenty  to  thirty  feet  deep.  The  surface 
of  the  swamp  is  decidedly  higher  than  the  inLmediately 


ORGANIC  AOENCIES.  87 

contiguous  country.  The  central  lake,  which  is  seven 
miles  in  diameter,  is  probably  the  remnant  of  a  once 
larger  lake,  as  just  explained.  Fig.  43  is  an  ideal  section 
illustrating  these  facts. 

Rate  of  Growth. — In  some  cases  the  increase  of  peat 
deposits  is  rapid.  In  Germany,  bogs  are  known  which 
have  formed  since  the  Eoman  invasion  ;  for  Eoman  roads 
are  traced  beneath  them,  and  stumps  and  logs  of  trees, 
felled  by  Roman  axes,  and  even  the  axes  themselves,  have 
been  found  at  the  bottom,  covered  with  from  ten  to  fifteen 
feet  of  peat.  The  bogs  have  been  formed  by  the  obstruc- 
tion of  drainage  caused  by  felling  the  trees.  Similarly, 
many  of  the  bogs  of  England  were  formed  at  the  time  of 
the  Norman  conquest,  by  the  felling  of  forests,  in  order 
to  exterminate  bands  of  Saxon  outlaws.  On  the  other 
hand,  in  the  great  peat-swamps,  where  the  accumulation 
is  strictly  by  growth  in  place,  the  increase  must  be  very 
slow,  perhaps  only  a  few  inches  per  century.  It  is  evi- 
dent, then,  that  the  rate  is  very  variable,  and  therefore 
no  safe  estimate  of  age  can  be  based  on  thickness. 

Section  of  a  Peat-Bog. — A  section  of  a  bog  reveals 
the  following  :  1.  Beneath  is  usually  a  clay  on  which  are 
often  found  the  stumps  and  roots  of  the  preceding  forest- 
growth.  The  under-clay  seems  necessary  to  hold  the 
water  without  which  peat  will  not  form.  2.  Above  this 
a  mass  of  black,  carbonaceous  matter,  structureless  to  the 
eye,  but  showing  its  vegetable  origin  to  the  microscope. 
3.  This  passes  by  gradations  through  imperfect  peat  into 
the  recently  fallen  leaves  and  branches,  and  the  still  grow- 
ing vegetation.  Now,  imagine  this  covered  with  mud  or 
sand,  deeply  buried,  and  subjected  to  great  pressure  for 
ages,  and  we  can  easily  see  that  it  would  become  con- 
verted into  a  coal-seam,  with  its  under-clay  full  of  roots 
and  stumps,  and  its  roof -shale  full  of  impressions  of  leaves 
and  flattened  stems. 

Alternation  of  Peat  with  River-Silt. — We  have  said 


88  DYNAMICAL  GEOLOGY. 

that  peat  occurs  in  the  river-swamps  and  deltas  of  great 
rivers.  It  is  easy  to  see,  therefore,  how  peat  deposits 
may  at  long  intervals  be  flooded  and  covered  with  river- 
silt,  and  again  reclaimed  and  covered  again  with  peat 
vegetation,  perhaps  many  times.  Now,  in  cutting  into 
the  delta  of  the  Mississippi,  several  layers  of  peat,  with 
interstratified  silts,  are  found.  The  resemblance  of  this 
to  a  series  of  coal-seams  on  a  small  scale  is  very  striking. 
It  is  by  observing  things  now  going  on  that  we  find  the 
key  for  interpreting  things  which  occurred  in  earlier  geo- 
logical times.     We  shall  apply  these  principles  in  Part  III. 

Drift-  Timber. 

But  there  is  another  way  in  which  vegetable  accumula- 
tions occur  now,  and  therefore  may  have  occurred  in 
previous  epochs.  Great  rivers  in  heavily  wooded  coun- 
tries, like  the  Mackenzie  and  the  Mississippi,  in  flood- 
times,  bring  down  large  quantities  of  drift-timber  gath- 
ered in  their  upper  courses,  and  accumulate  them  in  the 
form  of  rafts  at  their  mouths.  These  natural  rafts  are 
often  of  great  extent.  One,  at  the  entrance  of  the  Atcha- 
falaya,  near  the  head  of  the  delta  of  the  Mississippi,  was 
in  1838  ten  miles  long,  a  quarter  of  a  mile  wide,  and 
many  feet  thick.  Such  rafts  become  finally  water-logged, 
sink,  and  are  covered  up  in  a  river-silt.  Then  they  are 
slowly  changed  into  a  brownish,  cheesy  substance,  and 
doubtless  finally  into  lignite  or  coal.  Now,  in  cutting 
into  the  delta  deposit  of  the  Mississippi,  layers  of  drift- 
timber  are  met  with  which  is  undergoing  this  change. 
This  also  may  throw  light  on  the  formation  of  coal  and 
lignites. 

Section^  II. — Iroist  Accumulations. 

Every  one  must  have  observed  that  in  certain  boggy 
spots,  on  hillsides,  or  on  plains  at  the  foot  of  hills,  are 
found  reddish  deposits  of  iron  mixed  with  earth.     This 


ORGANIC  AGENCIES.  89 

form  of  iron  is  called  bog  iron-ore.  It  is  by  observing 
such  phenomena,  and  trying  to  find  out  how  they  are  pro- 
duced, that  we  may  expect  to  throw  light  on  the  forma- 
tion of  the  great  iron-beds  which  are  found  in  the  strata 
of  earlier  geological  times.  More  commonly  the  iron  is 
in  the  form  of  hydrated  ferric  oxide  {'^^Qfi^jZUfi)  but 
sometimes  of  ferrous  carbonate  (FeCOg). 

Mode  of  Formation. — Iron  has  a  very  strong  affinity 
for  oxygen,  as  is  shown  by  the  rapid  rusting  of  iron  when 
exposed  to  the  weather.  But  this  is  true,  not  only  of 
metallic  iron,  but  also  of  ferrous  oxide,  and  of  ferrous 
carbonate.  In  all  cases  it  runs  rapidly  into  the  condition 
of  highest  oxidation — Viz.,  ferric  oxide.  But,  although 
iron  has  so  strong  an  affinity  for  oxygen,  yet  2t,  portion  of 
the  oxygen  of  ferric  oxide  will  be  taken  away  from  it  by 
the  superior  affinity  of  organic  matter  in  a  state  of  decom- 
position. Thus,  ferric  oxide  (Fe^Og)  in  contact  with 
decomposing  organic  matter  will  be  reduced  to  ferrous 
oxide  (FeO),  which  then  readily  unites  with  carbonic 
acid  (CO3),  always  present  in  meteoric  waters,  and  forms 
ferrous  carbonate  (FeCOg).  Ferrous  carbonate  is  feebly 
soluble  in  water  containing  CO^. 

Now,  iron  is  a  very  abundant  substance,  but,  on  account 
of  its  affinity  for  oxygen,  it  exists  most  naturally  only  in 
the  form  of  ferric  oxide  ;  in  which  state,  therefore,  it  is 
almost  universally  diffused  as  a  red  or  yellow  coloring- 
matter  of  soil  and  rocks.  In  this  state,  though  abundant, 
it  is  unavailable  to  man.  But  organic  matter,  in  a  state 
of  decay,  is  everywhere  on  the  surface  of  the  ground. 
This  is  dissolved  by  rain-water,  and  sinks  into  the  earth. 
Therefore,  all  subterranean  water  contains  organic  matter 
in  solution.  Such  water,  percolating  through  red  soils 
or  red  rocks,  first  reduces  the  iron  io  ferrous  oxide,  then 
to  ferrous  carbonate,  then  takes  it  into  solution — i.  e., 
washes  it  out  of  the  soil  or  rock,  leaving  these  decolor- 
ized, then  comes  to  the  surface,  as  springs  containing 


90  DYNAMICAL  GEOLOGY. 

iron  carbonate  (chalybeate  springs).  This  is  where  we 
took  it  up  (page  75).  Then,  as  was  there  shown,  it  again 
comes  in  contact  with  air,  gives  up  CO^  and  retakes  oxy- 
gen, and  is  reconverted  into  ferric  oxide,  which,  being 
insoluble,  is  deposited. 

The  above  is  a  complete  explanation  of  the  accumula- 
tions of  ferric  oxide.  In  this  case  the  organic  matter  is 
consumed,  i.  e.,  changed  into  00,  and  HjO,  in  doing  the 
work  of  reduction  and  solution,  and  there  is  nothing  to 
prevent  the  iron  from  returning  to  the  condition  of  ferric 
oxide.  But,  if  there  be  an  excess  of  organic  matter,  as 
peat,  for  example,  in  the  place  where  the  deposit  occurs, 
then  the  iron  will  he  deposited  as  ferrous  carbonate, 
because  it  can  not  exist  in  the  form  of  ferric  oxide  in  the 
presence  of  decaying  organic  matter.  This  is  a  sufficient 
explanation  of  deposits  of  iron-carbonate. 

Familiar  Illustrations. — We  have  gone  so  far  into 
this  explanation  because  the  effects  of  water  containing 
00^  in  leaching  out  the  coloring-matter  of  soils  may  be 
observed  on  every  hand,  and  thus,  therefore,  afford  an 
excellent  field  for  cultivating  the  observing  power  of  the 
pupil. 

1.  If  a  dead  stump,  with  roots  ramifying  in  red  soil,  be 
examined,  it  will  often  be  observed  that  the  soil  is  bleached 
immediately  about  each  root.  This  is  because  water  con- 
taining organic  matter,  running  down  the  root,  leaches 
out  the  red  coloring-matter  of  the  soil. 

2.  In  every  railroad-cutting,  or  other  excavation  in  red 
soil,  it  will  be  observed  that  the  walls  of  every  fissure  in 
the  soil,  through  which  water  from  the  surface  descends, 
will  be  bleached  for  a  little  distance  on  each  side. 

3.  Bed  clays  exposed  to  view  by  excavations,  natural 
or  artificial,  are  often  variegated  or  marbled  with  irregu- 
lar streaks  and  spots  of  deeper  or  lighter  color.  This  is 
produced  by  irregular  percolations  of  water  containing 
organic  matter. 


ORGANIC  AGENCIES.  91 

4.  Even  in  the  most  intensely  red-clay  regions,  in 
wocrded  places  the  surface-soil,  for  a  foot  or  more  in 
depth,  is  bleached.  Water  containing  organic  matter 
from  the  surface  leaches  out  and  carries  down  the  color- 
ing iron  to  the  subsoil. 

5.  The  clay  of  uplands  may  be  yellow  or  red,  but  the 
clay  of  swamp-lands  is  always  Uuish.  This  is  because 
ferric  oxide,  which  is  the  red  or  yellow  coloring-matter, 
can  not  exist  in  the  presence  of  organic  matter,  abundant 
in  swamps,  but  is  reduced  to  ferrous  carbonate  and  its 
color  destroyed.  But,  if  such  blue  clay  be  burned  to 
brick  the  organic  matter  is  destroyed,  the  iron  is  peroxi- 
dized,  and  the  hrick  is  red. 

Section  III. — Lime  Accumulations. 

Lime  accumulations  are  made  mainly  by  corals  and  by 
shells.  We  shall  take  up  the  subject  under  these  two 
heads. 

Coral  Reefs  and  Islands. 

Although  corals  do  not  make  reefs  in  temperate 
regions,  and  therefore  the  process  can  not  be  observed  by 
every  one,  yet,  for  many  reasons,  the  subject  is  of  peculiar 
interest,  both  popular  and  scientific.  Coral  reefs  are  of 
peculiar  popular  interest— 1.  On  account  of  the  strange 
forms  and  gorgeous  beauty  of  the  animals  which  inhabit 
them.  2.  On  account  of  the  gem-like  beauty  of  the  isl- 
ands which  form  on  them.  3.  Because  a  large  area  is 
added  to  the  habitable  land-surface  by  the  agency  of 
corals  ;  and  especially,  4.  Because  the  largest  continuous 
body  of  land  thus  added  is  on  our  own  coast,  viz.,  in 
Florida.  5.  Because  of  the  great  dangers  to  navigation, 
especially  on  the  coast  of  Florida,  resulting  from  tfie 
presence  of  these  reefs.  The  considerable  town  of  Key 
West  owes  its  existence  wholly  to  the  wrecking  business. 


92  DYNAMICAL   GEOLOGY. 

There  are  also  peculiar  points  of  scuntific  interest.  To 
the  geologist  they  are  of  the  extremest  interest — >.  As 
agents  producing  immense  accumulations  of  limestone. 
2.  As  evidences  of  crust  movements  on  a  magnificent 
scale.     These  points  will  be  brought  out  as  we  proceed. 

It  is  a  common  idea — an  idea  which  has  passed  into 
popular  literature,  and  is  difficult  to  eradicate — that 
corals  and  coral  reefs,  like  the  hills  and  galleries  of  ants, 
are  built  slowly  by  the  cooperative  labor  of  millions  of 
little  insects.  It  becomes  necessary,  therefore,  to  explain 
somewhat  fully  the  manner  in  which  a  reef  is  really 
formed. 

A  Simple  Polyp. — Fig.  44  represents  an  ordinary  soft 


Fig.  44.— Simplified  figure  of  an  actiuia. 

polyp  {Actinia — sea-anemone) ^  somewhat  simplified,  such 
as  may  be  seen  clinging  to  rocks  or  piers  on  our  sea-shores 
almost  anywhere.  Their  structure  is  diagrammatically 
shown  in  section  (Fig.  45).  As  seen  by  these  figures  the 
creature  may  be  compared  to  a  hollow,  fleshy  cylinder, 
closed  at  both  ends  like  a  yeast-powder  can.  The  lower 
may  be  called  the  /oo^-disk,  the  upper  the  mo?«^7i-disk. 
The  edge  of  the  mouth-disk  is  surrounded  by  hollow  ten- 
tacles, t  t,  which  open  into  the  hollow  cylinder.  In  the 
center  of  the  mouth-disk  is  the  mouth,  m,  and  below  it 
hangs  the  stomach,  s,  reaching  about  half-way  down.     At 


ORGANIC  AGENCIES. 


93 


the  lower  end  of  the  stomach  is  the  pylorus,  which  may 
be  opened  and  shut  like  a  second  mouth.  Running  from 
the  outer  wall,  and  converging  toward  the  axis,  are  many 
partitions,  p  p,  some  of  which  reach  the  stomach  and 
hold  it  steadily  in  the  axis,  but  below  the  stomach  termi- 
nate in  free,  scythe-like  edges.  These  converging  par- 
titions divide  the  body  cavity  into  a  number  of  trian- 


FiG.  45.— Ideal  section,  vertical  and  horizontal,  siiowing  structure  :  t,  tentaclee  ; 
s,  stomach  ;  ^>,  p,  partitions. 


gular  apartments,  which,  however,  are  in  free  communi- 
cation with  each  other  below  the  stomach.  Besides  the 
main  partitions  spoken  of,  there  are  very  many  smaller 
ones  which  do  not  reach  so  far  as  the  stomach.  The 
whole  structure  may  be  briefly  summarized  by  tracing  the 
course  of  the  food.  Food  is  taken  by  the  tentacles,  put 
into  the  mouth,  and  passes  into  the  stomach.  After 
digestion,  whatever  is  refuse  is  thrown  back  through  the 
mouth,  and  the  digested  food  is  dropped  through  the 
pylorus  into  the  general  hall  below  the  stomach,  and 
there  mixed  with  sea-water  and  circulated  through  all  the 
apartments. 

Simple  Coral,  or  Stone  Polyp. — Now,  a  simple  coral 
has  a  similar  structure,  except  that  stony  matter  (lime 


94 


DYNAMICAL   GEOLOGY. 


carbonate)  is  deposited  in  the  lower  part  as  high  as  about 
the  region  of  the  stomach,  as  shown  in  Fig.  46.  When 
the  animal  seems  to  disappear,  it  only  withdraws  the  soft 
upper  parts  within  the  stony  lower  part.  But  the  stony 
material  is  everywhere  within  the  living  organic  matter 
and  covered.     When  the  living  organic  matter  is  taken 


Pig.  46.— Ideal  section  of  a  single  living  coral.    The  shaded  portion  contains 
carbonate  of  lime. 

away,  as  in  dead  corals,  then  we  have  only  the  radiated 
structure  of  the  lower  part  in  stone.  This  is  well  shown 
in  Fig.  47,  a  and  d.  The  corals  which  form  reefs,  how- 
ever, are  individually  extremely  small. 


Fig.  47. — a,  stony  part  of  a  single  coral ;  ft,  section  of  same,  showing  structure. 

Compound  Coral,  or  Coralluiu. — Many  lower  ani- 
mals, like  plants,  have  the  power  of  reproducing  by  buds. 
If  the  buds  separate,  they  form  distinct  individuals  ;  but 


ORGANIC  AOENCIES.  95 

if  they  remain  attached,  then  a  compound  animal  is 
formed,  composed  of  many  individuals,  united  together 
precisely  as  a  tree  is  formed  of  many  buds,  each  of  which 
is  in  some  sense  an  individual,  and  capable  of  independent 
life.  In  the  compound  coral  each  bud  has  its  own  tenta- 
cles, mouth,  stomach,  partitions,  and  other  organs  neces- 
sary for  life,  and  yet  all  are  organically  connected,  and 
each  feeds  for  all.  There  is,  therefore,  a  sort  of  individ- 
uality in  the  aggregate,  but  a  more  decided  individuality 
in  each  bud. 

The  form  of  the  aggregate  depends  on  the  mode  of 
budding.  If  the  buds  grow  into  branches,  then  there  is 
formed  a  tree-coral  (Fig.   48) ;  but  if  the  buds  do  not 


Fig.  48.— Madrepora,  a  tree-coral. 

separate,  but  remain  connected  to  their  ends,  and  form 
new  buds  in  the  intervening  spaces,  then  they  form  a  head- 
coral  (Fig.  49).  There  are  all  gradations  between  these 
extremes.  Coral-trees  are  often  six  to  eight  feet  high,  so 
that  one  may  literally  climb  among  the  branches.  Coral- 
heads  form  hemispherical  masses  fifteen  to  twenty  feet  in 
diameter.  In  either  case  the  aggregate  consists  of  hun- 
dreds of  thousands  of  individuals  ;  in  either  case,  also, 
the  living  organic  matter  is  confined  to  the  superficial 
portion,  one  quarter  to  one  half  an  inch  thick.     As  in 


96  DYNAMICAL  OEOLOOY. 

case  of  a  tree,  so  in  corals,  life  passes  continually  outward 
and  upward,  leaving  the  middle  parts  dead,  and,  in  fact, 
wholly  composed  of  mineral  matter  (lime  carbonate),  re- 


FiG.  49. — Astrea,  a  liead-coral. 

taining,  however,  the  peculiar  structure  given  it  while 
permeated  with  living  matter. 

Coral  Forests. — Corals,  however,  reproduce  also  hy 
eggs.  These  are  formed  within,  below  the  stomach,  ex- 
truded through  the  mouth,  and  having,  like  the  eggs  of 
many  lower  animals,  the  power  of  locomotion,  swim  away 
and  settle  to  the  bottom,  where,  if  conditions  are  favor- 
able, they  form  single  corals,  which,  by  budding,  soon  form 
coral-trees  or  coral-heads.  In  this  way  a  coral  forest  or 
grove  is  formed,  and  spreads  in  all  directions  as  far  as 
favoring  conditions  allow  (Fig.  50). 

Coral  Reefs. — But  coral  forests  are  not  yet  coral  reefs. 
These  are  formed  by  the  growth  and  decay  on  the  same 
spot  of  countless  generations  of  coral  forests.  Each 
generation  in  its  death  leaves  its  limestone  behind  ;  and 
thus  the  coral  ground  rises  or  is  built  up  without  limit 
except  by  reaching  the  sea-level.  As  a  peat-bog  is  formed 
bv  the  accumulated  remains  of  successive  generations  of 


OROANTO  AGENCIES. 


97 


plants  growing  and  dying  on  tlie  same  spot,  so  a  reef  is 
similarly  formed  by  successive  generations  of  corals.     As 


Fig  .  50.  —Corals  in  the  Great  Barrier  Reef,  Australia. 


peat-ground  may  rise  above  the  surrounding  country,  so 
a  coral  reef  rises  far  above  the  surrounding  sea-bottom. 
As  peat  represents  so  much  carbon  taken  from  the  air 
and  added  to  the  ground,  so  a  reef  represents  so  much 
carbonate  of  lime  taken  from  the  sea-water  and  added  to 
the  sea-bottom.  The  limestone  thus  formed  by  the 
broken  remains  of  corals  cemented  together  is  called  the 
reef-rock.  Thus  a  reef  is  a  submarine  bank  composed  of 
reef-rock,  crowned  with  the  present  generation  of  living 
corals. 

Coral  Islands. — But  e\en  coral  reefs  are  not  yet  coral 
islands,  since  corals  can  not  grow  above  the  sea-level. 
Coral  islands  are  made  by  the  action  of  waves.     Waves 

Lk  Conte,  Geol.  7 


98  DYNAMICAL  GEOLOGY, 

will  form  islands  on  any  kind  of  submarine  bank  when 
the  water  is  shallow  enough  for  the  waves  to  touch  and 
chafe  the  bottom.  When,  therefore,  the  reef  rises  nearly 
to  the  surface,  the  beating  waves  will  break  off  coral- 
trees,  coral-heads,  and  even  masses  of  the  reef -rock. 
Great  masses  are  thus  rolled  up  on  the  inner  side  of  the 
reef,  and  form  a  nucleus  about  which  other  masses  gather. 
Among  these  larger  masses  smaller  masses  are  thrown, 
then  finely  comminuted  coral  limestone  (coral  sand)  is 
sifted  among  these,  and  the  whole  is  cemented  into  a  solid 
rock  by  carbonate  of  lime  in  tlie  sea-water.  The  island 
rock,  therefore,  is  a  hreccia  of  coral  limestone^  as  shown 
in  Fig.  61o     The  island  thus  formed  is  at  first  barren 


Fig.  51.— Ideal  section  across  a  coral  island  ;  ;,  l^  sea-level ;  R,  living  reef ;  G.B.B.^ 
coral  reef -rock. 

rock  ;  but,  slowly,  seeds  are  brought  by  waves  and  wind  ; 
it  becomes  covered  with  vegetation,  and  inhabited  by 
animals  and  by  man. 

Thus  we  have  traced  the  whole  process,  and  find  no 
evidence  of  purpose  or  will,  much  less  the  admirable  vir- 
tues of  perseverance  and  industry,  often  attributed  to 
them.  It  is  a  pity  to  spoil  a  moral  ;  but  truth  is  the  best 
moral. 

Conditions  of  Growth. — Reef-building  corals  do  not 
grow  over  the  whole  sea-bottom,  nor  in  all  oceans.  They 
are  strictly  limited  by  certain  conditions  : 

1.  They  will  not  grow  where  the  mean  winter  tempera- 
ture of  the  ocean  is  less  than  68°  Fahr.  This  condition 
confines  them  mostly  to  the  tropics.     The  most  notable 


ORGANIC  AGENCIES,  •  99 

apparent  exception  to  this  is  in  the  North  Atlantic.  On 
the  coast  of  Florida  and  the  Bahamas  reefs  occur  as  far 
as  28°  and  on  the  Bermudas  as  far  as  32°  north  latitude. 
But  this  is  because  the  temperature  of  68°  is  carried 
northward  by  the  warm  waters  of  the  Gulf  Stream. 

2.  Reef-building  corals  will  not  grow  at  a  greater  depth 
than  about  one  hfindred  feet.  This  condition  confines 
them  to  submarine  banks,  and  especially  to  shore  lines. 
In  tropic  seas  corals  build  all  along  the  shore,  and  as  far 
out  as  the  depth  will  allow.  Hence  results  the  usual 
linear  form  of  reefs. 

3.  They  require,  also,  clear  salt  water,  and  are  killed 
by  fresh  water  and  by  mud.  They  will  not  grow,  there- 
fore, along  flat,  muddy  shores  where  the  waves  chafe  the 
bottom  and  stir  up  mud.  Also,  if  a  reef  is  formed  along 
a  shore-line,  there  will  be  breaks  in  the  reef  oif  the 
mouths  of  rivers,  the  corals  being  prevented  from  growing 
there  partly  by  the  freshness  of  the  water,  and  partly  by 
the  mud  brought  down  by  the  river. 

4.  Corals  grow  best  where  they  are  beaten  by  the 
waves — viz.,  on  the  outer  portion  of  the  reef.  Some  spe- 
cies, indeed,  love  the  still  water  on  the  inner  side  of  the 
reef,  but  the  strong,  reef-building  species  thrive  under 
the  effect  of  the  dashing  waves,  and  will  even  build  up- 
ward in  the  face  of  waves  that  would  wear  away  a  granite 
wall.  The  corals  are  broken,  indeed,  and  worn,  but 
growth  more  than  makes  up  for  the  wear.  This  is 
because  the  crowded  life  on  the  reef,  both  of  corals  and 
of  animals  of  all  kinds  feeding  on  the  corals,  rapidly 
exhausts  the  water  of  its  oxygen  and  replaces  it  with 
carbonic  acid,  and  thus  renders  it  unfit  to  support 
life.  But  the  chafing  and  foaming  of  the  breakers  dis- 
charges the  CO,  to  the  air  and  restores  the  oxygen.  It 
is  exactly  like  the  ventilation  so  necessary  for  air-breath- 
ing animals. 

All  these  conditions  refer  only  to  reef-building  species. 


100 


DYNAMICAL  GEOLOGY 


Some  species  of  corals  live  at  great  depths  and  in  high 
latitudes. 

Description  of  Pacific  Coral  Reefs  and  Islands. 

— There  are  in  the  Pacific  two  very  distinct  kinds  of 
islands — viz.,  volcanic  islands  and  coral  islands.  The 
former  are  high,  bold,  rocky,  and  often  of  considerable 
size ;  the  latter  low,  wave-formed.  We  will  suppose  the 
preexistence  of  volcanic  islands,  and  proceed  to  show  how 
coral  reefs  and  islands  are  formed  about  them. 

Pacific  reefs,  then,  are  of  three  principal  kinds — ^viz., 
fringing  reefs,  barrier  reefs,  and  circular  reefs  or  atolls. 

1.  Fringing  Reefs. — These  grow   along  any   shore- 


PiG.  58.— Perspective  view  of  volcanic  island  and  fringing  reef. 

line,  but  the  most  common  and  interesting  are  those  about 
volcanic  islands.  Suppose,  then,  a  high  volcanic  island  in 
the  midst  of  the  sea.  Around  such  an  island  corals  will 
build,  limited  outward  by  increasing  depth,  limited  inward 


Pig.  53.— Ideal  section  of  volcanic  island  and  fringing  reef;  s.p.^  shore  platform  | 
cp,  coral  platform. 

by  shore-line  and  upward  by  sea-level,  thus  forming  a 
submarine  platform  clinging  close  to  the  island  like  a 


ORGANIC  AGENCIES, 


101 


fringe.  The  existence  and  extent  of  such  a  reef  are 
revealed  by  the  snow-white  sheet  of  breakers  which  sur- 
rounds the  island  like  a  snowy  girdle  (Fig.  52).  Off  the 
mouths  of  large  rivers  breaks  in  the  reef  will  occur.  Fig. 
53  is  an  ideal  section  showing  the  coral  platform,  cp,  cp. 

So  much  for  the  agency  of  corals.  The  waves  now 
break  off  fragments  from  the  outer  part  of  the  reef  and 
pile  them  up  on  the  inner  part  against  the  land,  and  thus 
form  a  low,  level  sliore  platform,  s.p.,  s.p.,  above  the 
sea-level.  Thus  we  have  first  the  slope  of  the  volcanic 
island ;  then  the  shore-platform  of  coral  debris ;  then 
the  s.ubmarine  platform  of  living  corals  ;  and,  finally,  the 
deep  water.  In  this  case  there  is  no  coral  island,  but 
only  a  coral  addition  to  the  volcanic  island. 

2.  Barrier   lieefs. — About  the  volcanic  island  there 


Fig.  54.— Perspective  view  of  volcanic  island  and  barrier. 

may  be  little  or  no  fringing  reef,  but  at  a  distance  of 
five,  ten,  or  fifteen  miles  away,  in  deep  water,  there  rises 


Fig.  55.— Section  of  volcanic  island  and  barrier. 


a  line  of  reef  like  a  great  rampart  surrounding  the  island, 
and,  as  it  were,  protecting  it  from  the  attacks  of  the  sea. 


102 


DYNAMICAL  GEOLOOY. 


The  position  of  the  reef  is  shown  by  a  snowy  girdle  of 
breakers,  within  which,  like  a  charmed  circle,  there  is 
calm  sea  in  the  wildest  storm.  Between  the  reef  and  the 
island  there  is  a  ship-channel,  often  twenty  or  thirty 
fathoms  deep.  Through  breaks  or  tidal  ways  in  the  reef, 
ships  enter  and  find  good  harbor  in  the  channel.  If  it 
were  not  for  the  action  of  the  waves,  this  would  be  all, 
but  the  beating  waves  form  little  coral  islands  on  the 
reef,  so  that,  instead  of  a  continuous  snowy  girdle,  it  is 
such  a  girdle  gemmed  on  the  inner  edge  with  a  string  of 
green  islets.  By  sounding  it  is  found  that  the  inner  slope 
of  the  reef  is  gentle,  but  the  outer  slope  is  very  steep,  and 
rapidly  passes  into  abyssal  depth.  All  these  facts  are  shown 
in  the  perspective  view.  Fig.  54,  and  the  section.  Fig.  55. 
3.  Circular  Reefs,  or  Atolls. — These  are  the  most 
remarkable  of  all.  In  this  case  there  is  no  volcanic  island 
or  preexisting  land  of  any  kind  apparent,  as  a  nucleus 


Fig.  56.— Perspective  view  of  an  atoll. 

for  the  growth  of  corals.  The  reef  seems  to  have  been 
built  up  from  abyssal  depth,  in  an  irregular  circular 
form,  inclosing  a  lagoon  of  still  water  in  the  midst  (Fig. 
56).  The  position  of  the  reef,  r,  r,  is  shown  by  a  circle 
of  snowy  foam  inclosing  and  protecting  a  harbor  of  still 
water.  Through  breaks  in  the  reef-circle  ships  may 
enter  and  find  safe  anchorage.  The  lagoon  is  ten,  twenty, 
thirty,    or  even   fifty  miles  in   diameter,  and   thirty  or 


ORGANIC  AGENCIES. 


103 


forty  fathoms  deep.  By  sounding  it  is  found  that  the 
inner  reef-slope  is  gentle,  but  the  outer  very  steep,  so 
that  at  a  distance  of  a  mile  more  than  a  mile  depth  has 
been  found   (Fig.   57).     Thus  far,  the  action  of   corals 


Fig.  57. — Section  of  an  atoll. 

alone.  Now  add  the  action  of  waves,  and  the  snowy  ring 
is  gemmed  on  the  inner  edge  with  small  green  islets.  All 
these  facts  are  shown  in  Figs.  56  and  57. 

4.  Closed  Lagoons  and  Lagoonless   Islands.  —  In 
the  typical  atoll  the  reef-circle  is  large,  and  only  dotted 


Pro.  88.— Map  view  of  closed  lagoons  and  lagoonless  islands. 

with  small  islets,  but  in  small  atolls  the  land  is  more  con- 
tinuous (Fig.   58,  a),    or    entirely   continuous,    but   the 


104  DYNA3nCAL  GEOLOGY. 

lagoon  open  to  the  sea  on  one  side  (Fig.  58,  h),  or  the 
lagoon  maybe  entirely dose^  (Fig.  58,  c),  or  the  ring  may 
close  in  upon  itself  so  as  to  abolish  the  lagoon  (Fig.  58,  d). 
These  are  so  different  from  the  typical  atoll  that  they 
may  be  considered  a  fourth  class. 

Theory  of  Barriers  and  Atolls. 

Fringing  reefs  need  no  theory.  Corals,  finding  the  con- 
dition of  suitable  depth  along  the  shore,  build  upward 
to  the  sea-level  and  outward  to  the  depth  of  one  hundred 
feet,  and  thus  form  a  coral  platform  clinging  to  the  orig- 
inal island.  But  barriers  seem  at  first  sight  to  form  far 
from  land  in  abyssal  depth ;  and  atolls  seem  to  form  in 
deep  sea  without  any  island-nucleus.  These  facts  seem  to 
violate  the  conditions  of  coral  growth.  How  are  they 
explained  ?  The  most  probable  explanation  was  first 
given  by  Mr.  Darwin. 

Darwiu's  Subsidence  Theory. — According  to  Dar- 
win, every  reef  began  as  a  fringe,  and  would  have  remained 
so  if  the  floor  of  the  ocean  had  remained  steady.  But,  in 
all  the  region  of  barriers  and  atolls,  the  ocean-floor  has 
slowly  subsided,  carrying  all  the  volcanic  islands  with  it 
downward.  Now,  if  the  subsidence  had  been  more  rapid 
than  the  coral  ground  could  rise  by  accumulations  of  debris 
of  successive  generations,  then  the  corals  would  have  been 
carried  below  the  depth  of  one  hundred  feet  and  drowned. 
But  the  subsidence  was  not  faster  than  the  coral  ground 
could  be  built  up.  Therefore  the  corals  building  upward, 
as  it  were,  for  their  lives,  kept  their  heads  at  or  near  the 
surface.  But  the  reef,  building  up  nearly  at  the  same 
place,  while  the  volcanic  island  grew  smaller,  it  is  evident 
that  the  latter  would  be  separated  more  and  more  from 
the  reef.  When  the  island  was  down  waist-deep,  the  reef 
was  a  barrier  ;  when  down  head-under,  it  became  an  atoll, 
the  reef  representing  nearly  the  outline  of  the  original 
base  of  the  volcanic  island.     We  said  nearly,  but  not  per- 


ORGANIC  AGENCIES. 


105 


fectly.  The  corals  do  not  build  up  perpendicularly,  but 
in  a  steep  slope.  The  barrier,  and  much  more  the  atoll, 
is  therefore  smaller  than  the  original  fringe.  If,  there- 
fore, the  subsidence  continues,  the  atoll  will  grow  smaller 
and  smaller,  the  separate  islets  will  close  together,  join 
each  other,  and  finally  close  the  lagoon.  Then  the  lagoon 
will  close  in  upon  itself  and  form  the  lagoonless  island, 
and,  last  of  all,  this  also  will  probably  disappear. 

As  corals  grow  best  on  the  outside  of  the  reef,  they 
will  not  occupy  the  channel  formed  by  recession  of  the 
volcanic  island  ;  or,  if  they  do,  they  are  soon  drowned  out 
by  subsidence.  The  channel,  however,  in  case  of  barriers, 
or  lagoon  in  case  of  atolls,  will  be  partly  filled  by  debris 
carried  into  it  in  both  cases  from  the  reef,  and  in  the  case 
of  barriers  also  from  the  volcanic  islg,nd.     Fig.  59  is  an 


Fig.  59.— Ideal  section  diagram  showing  the  formation  of  an  atoll  ;  l'\  l'\  sea-level 
when  reef  was  a  fringe  ;  l\  I',  when  it  was  a  barrier,  and  /,  I,  the  present  sea- 
level. 

ideal  section  embodying  all  these  facts.  In  this  figure, 
for  convenience  of  illustration,  instead  of  the  sea-bottom 
sinking,  the  sea-level  is  represented  as  rising. 

Murray's  Theory. — The  subsidence  theory,  however, 
is  not  now  universally  accepted.  Agassiz  first  showed,  by 
study  of  the  reefs  of  Florida  in  1851,  that  barriers  are 
formed  without  subsidence.  Murray,  traveling  in  the 
same  region  as  Darwin,  concluded  that  both  atolls  and  bar- 
riers may  be  formed  without  subsidence.  He  supposes 
that  atolls   are  built  up   by  corals  on  banks  previously 


106  DYNAMICAL  GEOLOGY. 

formed  by  other  agencies,  the  corals  growing  only  on  the 
outer  margin  of  the  bank,  because  they  find  the  best  condi- 
tions of  growth  there.  Barriers,  he  supposes,  are  fringes 
separated  from  the  encircled  island  by  dying  out  of  the 
corals  on  the  landward  side  and  extension  on  the  seaward 
side  of  the  reef. 

It  is  probable  that  atolls  and  barriers  are  formed  in 
several  ways,  but  under  present  lights  it  is  not  improb- 
able that  many,  if  not  all,  atolls  are  formed  as  Darwin 
supposes.  We  will  assume  that  every  atoll  marks  the 
place  of  a  drowned  volcanic  island. 

Area  of  Subsidence. — The  area  of  the  subsiding  sea- 
floor  is  6,000  miles  long  and  3,000  miles  wide.  It  is  prob- 
ably not  less  than  12,000,000  square  miles,  or  greater  than 
the  whole  North  American  Continent. 

Area  of  Laud  Lost. — This  must  not  be  confounded 
with  the  sinking  area.  The  sinking  area  is  the  whole 
sea-floor,  over  millions  of  square  miles  ;  the  land  known 
to  have  been  lost  is  only  the  volcanic  islands  which  once 
overdotted  this  area.  This  is,  of  course,  small  in  com- 
parison. Estimated  by  the  circles  inclosed  by  atolls, 
Dana  makes  it  50,000  square  miles.  It  is  doubtless,  how- 
ever, much  more  than  this.  For — 1.  This  estimate  takes 
no  account  of  barriers,  but  all  the  araa  between  a  barrier 
and  the  shore-line  is  also  lost.  Now,  along  the  Australian 
coast,  for  1,100  miles,  there  is  a  barrier  thirty  to  forty 
miles  distant.  This  alone  would  make  33,000  square 
miles  lost  for  this  one  barrier.  We  may  with  confidence, 
therefore,  double  the  estimate.  But,  2.  Atolls  themselves, 
as  already  shown,  are  smaller,  and  closed  lagoons  and 
lagoonless  islands  very  much  smaller  than  original  volcanic 
islands.  And,  3.  In  the  middle  of  the  coral  region  there 
is  a  blank  area  of  several  million  square  miles,  in  whicli 
there  are  no  islands  of  any  kind.  Many  islands  probably 
went  down  here  and  left  no  sign,  because  they  went  down 
too  rapidly  and   the  corals   were  drowned.     Putting  all 


ORGANIC  AGENCIES.  107 

these  facts  together,  it  seems  probable  that  several  hun- 
dred thousand  square  miles  of  volcanic  land  have  been 
lost.  Of  this  only  a  small  fraction  has  been  recovered  by 
the  action  of  corals  and  waves. 

Amount  of  Vertical  Subsidence. — This  may  be 
roughly  estimated  in  many  ways  :  1.  Soundings  a  little 
way  oif  barriers  have  reached  2,000  feet,  and  oif  atolls 
7,000  feet.  2.  The  average  slope  of  volcanic  islands  of 
the  Pacific  is  about  8°,  but,  taking  it  even  as  low  as  5°,  a 
barrier  ten  miles  from  shore  would  indicate  a  subsidence 
of  4,500  feet.  (D  B  =  A  D  .  tan  A.)  But  barriers  are 
found  at  much  greater  distances  than  ten  miles.  3.  The 
average  height  of  volcanic  islands  of  the  Pacific  in  non- 


FiG.  60.— t?,  volcanic  island  ;  ^-1,  shore    line ;  Z>,  place  of  barrier  ;  AB^  elope  of 
bottom,  5". 

subsiding  areas  is  6,000  to  10,000  feet.  Now,  every  atoll 
represents  such  an  island,  entirely  submerged,  and  every 
closed  lagoon  the  same  deeply  submerged.  But  it  is  very 
improbable  that  none  of  these  reached  the  average  of  those 
remaining.  Taking  all  these  facts  together,  it  is  probable 
that  the  extreme  subsidence  is  not  less  than  10,000  feet. 

Amount  of  Time  Involved. — It  is  evident  that  the 
rate  of  sinking  can  not  have  been  greater  than  the  rate  of 
coral  ground-rising  ;  otherwise  the  corals  would  have  been 
drowned.  Again,  the  rate  of  ground-rising  is  far  less  than 
the  rate  of  coral-prong  growth.  If  the  annual  growth  of 
all  the  prongs  were  taken,  ground  to  powder,  and  strewed 
over  the  area  shaded  by  the  coral  branches,  it  would  give 
the  annual  rising  of  the  ground.  It  is  evident  that  this 
would  be  very  small  in  comparison  with  the  growth  of  the 
prongs.  In  addition  to  this,  it  must  be  remembered  that 
large  spaces  of  a  coral  reef  are  bare.     Taking  all  these 


108  DYNAMICAL   OEOLOOY, 

things  into  consideration,  it  has  been  estimated  that  one 
quarter  to  one  half  inch  per  annum  is  a  large  estimate  of 
rate  of  ground-rising.  The  subsidence  can  not  be  greater 
and  may  be  much  less  than  this.  At  this  rate  a  subsi- 
dence of  10,000  feet  would  require  250,000  to  500,000 
years.  The  whole  of  this,  however,  must  not  be  accred- 
ited to  the  present  geological  epoch.  It  probably  extends 
back  into  the  Tertiary. 

Geological  Application, 

There  are  several  points  in  the  preceding  discussion 
which  throw  important  light  upon  the  structure  and  his- 
tory of  the  earth.  1.  We  have  here  examples  of  lime- 
stone rock,  formed  by  coral  agency  over  millions  of  square 
miles,  and  in  places  many  thousand  feet  thick.  For  not 
only  is  limestone  formed  on  the  site  of  the  reefs  (reef -rock), 
but  the  fine  coral  debris  is  carried  by  waves  and  currents 
and  strewed  over  the  whole  intervening  space.  We  find 
thus  a  key  to  the  extensive  deposits  of  limestone  formed 
in  previous  geological  times.  2.  The  kind  of  rock  formed 
also  deserves  attention.  The  reef-rock  is,  in  some  parts, 
a  coral  breccia;  in  other  parts  it  consists  of  rounded 
granules,  cemented  together  (oolite).  In  the  deep  sea  of 
the  intervening  spaces,  the  bottom  ooze  is  Sbfine  coral  mud, 
which,  dried,  looks  much  like  chalk,  and  by  some  has 
(  been  supposed  to  be  indeed  the  modern  representative  of 
chalk  ;  but  more  probably,  it  hardens  into  a  compact 
limestone.  Now,  in  limestones  of  previous  geological, 
epochs,  we  find  similar  structures  ;  i.  e.,  extensive  fine 
limestone,  with  areas  of  coarse  coral  breccia  or  of  oolites. 
We  are  thus  able  to  determine  the  position  of  old  coral 
seas  and  the  lines  of  old  coral  reefs,  even  though  they  are 
now  occupied  by  mountain-ranges,  as  in  the  case  of  the 
Jura  Mountains  (Heer).  3.  Lastly,  we  have  here  ex- 
amples of  movements  of   the    earth^s  crust    on   a  grand 


ORGANIC  AGENCIES.  109 

scale — on  a  scale  commensurate  with  the  formation  of 
continents  and  ocean-bottoms.  The  phenomena  of  coral 
reefs  show  a  down-sinking  of  the  mid-Pacific  bottom  of 
several  thousand  feet,  and  over  an  area  of  many  million 
square  miles.  This  lias  been  going  on  through  later  geo- 
logical times,  and  is  probably  still  progressing.  Now,  so 
wide-spread  a  downward  movement  must  have  its  correla- 
tive in  an  upward  movement  somewhere  else.  It  seems 
probable  that  we  find  it  in  the  upheaval  of  the  western 
half  of  the  American  Continent,  both  North  and  South. 
It  is  well  known  that  during  the  whole  later  Tertiary,  even 
to  the  present  time,  the  western  part  of  North  America, 
especially  the  plateau  region,  has  been  slowly  rising,  the 
extreme  rise  being  nearly  20,000  feet.  As  it  rose,  the 
general  erosion  became  greater  and  the  caflons  cut  deeper 
and  deeper.  So  that  the  down-sinking  of  the  Pacific  bot- 
tom, the  upheaval  of  the  plateau  region,  and  the  cutting 
of  the  wonderful  caflons  of  that  region,  are  probably  all 
connected  with  each  other. 


REEFS  AND  KEYS  OF  FLORIDA.   ' 

The  reefs  of  Florida  deserve  separate  and  special  treat- 
ment, not  only  because  they  are  on  our  own  coast,  but  also 
because  they  are  in  some  important  respects  entirely  pecu- 
liar :  1.  In  the  Pacific,  barrier-reefs  are  always  the  result , 
and  the  sign  of  subsidence.  In  Florida,  on  the  contrary, 
we  have  barrier-reefs  where  there  has  been  no  subsidence. 
2.  In  the  Pacific,  corals  do  not  add  to  the  previously  ex- 
isting land-surface  ;  on  the  contrary,  they  only  recover  a 
small  fraction  of  a  lost  land-surf  ace.  But  in  Florida  there 
has  been  apparently  no  loss,  but  a  constant  growth  of  land- 
surface  under  the  action  of  corals,  assisted  by  waves  and 
other  agents,  as  we  shall  presently  explain.  Attention 
has  not  been  hitherto  sufficiently  drawn  to  the  entire 
uniqueness  of  these  reefs. 


1 


110 


DYNA3IICAL   GEOLOOY- 


Description  of  Reefs  and  Vicinity. — Fig.  61,  A,  is 
a  map  of  Florida,  its  keys,  reefs,  etc.,  and  Fig.  61,  B,  is  a 
section  of  the  same  along  the  line  N  S,     The  southern 


Pig.  6J.— Map  and  section  of  Peninsula  and  Keys  of  Florida.    In  both,  a  =  coast 


keys 


reef  ;  e  =  everglade  ;  e'  =  shoal  water  ;  e"  =  ship-channel 


GS  =  Gulf  Stream. 


coast  of  Florida,  «  «,  is  a  ridge  of  limestone,  twelve  to 
fifteen  feet  high,  inclosing  a  swamp  called  the  Everglades, 
e,  only  one  to  two  feet  above  the  sea-level,  covered  with 


OROANIC  AGENCIES.  HI 

fresh  water,  overgrown  with  vegetation,  and  overdotted 
with  higher  spots,  called  hummocks.  Going  south  from 
the  coast,  the  next  thing  that  attracts  attention  is  a  line 
or  string  of  limestone  islands  (keys)  a  a,  stretching  in  a 
curve  from  Cape  Florida  to  the  Tortugas,  a  distance  of 
one  hundred  and  fifty  miles.  Between  these  and  the 
southern  coast  is  an  extensive  shoal,  almost  a  mud-flat, 
navigable  only  to  small  fishing-craft.  The  width  of  this 
shoal  is  thirty  to  forty  miles.  It  is  overdotted  with  small, 
low,  mud  islands,  overgrown  with  mangrove-trees,  and 
entirely  different  from  the  true  keys.  Outside  of  the  line 
of  keys,  and  separated  from  it  by  a  ship-channel,  five  to 
six  miles  wide  and  three  to  four  fathoms  deep,  is  a  con- 
tinuous line  of  living  reef,  a"  a".  On  this,  by  the  action 
of  the  waves,  a  few  small  islands  have  commenced  to 
form.  Outside  of  all  sweep  the  deep  waters  of  the  Gulf 
Stream,  G  S. 

Formed  by  Coral  Agency. — Now,  the  whole  area 
thus  described  is  a  recent  coral  formation,  and  has  been 
added  to  Florida  in  recent  geological  times.  The  proof 
of  this  is  complete. 

First  :  On  the  living  reef,  islands  have  just  commenced 
to  form.  Some  are  yet  only  a  collection  of  large  coral 
fragments,  the  nucleus  of  an  island.  Some  are  more 
compacted  by  smaller  fragments  thrown  in  among  the 
larger.  Some  are  small  but  perfect  islands — i.  e.,  coral, 
sand,  and  mud  have  been  thrown  upon  and  completely 
buried  the  large  masses.  But  none  of  these  are  yet 
clothed  with  vegetation,  much  less  inhabited  by  animals 
and  man.  Next  come  the  larger  inhabited  islands  of  the 
line  of  keys.  On  cutting  into  these,  the  same  structure 
as  described  above  is  revealed.  Undoubtedly  these  are 
a  string  of  wave-formed  coral  islands,  and  here  was  once  a 
line  of  living  reef ;  but  the  corals  have  long  ago  died,  be- 
cause cut  off  from  the  open  sea  by  the  formation  of  another 
reef  farther  out.     Next  comes  the  southern  coast.     Ex- 


112  DYNA3nCAL   GEOLOGY. 

amination  of  this  reveals  the  same  structure  precisely. 
Here,  then,  was  the  place  of  a  still  earlier  reef. 

Brief  History  of  the  Process. — There  was,  there- 
fore, a  time  when  the  north  shore  of  the  Everglades  {d, 
section,  Eig.  61,  B)  was  the  southern  shore  of  Elorida. 
At  that  time  the  place  of  the  present  southern  coast  was 
occupied  by  a  living  reef.  On  this  reef  coral  islands  were 
formed,  which  gradually  coalesced  into  a  continuous  line 
of  land,  the  shoal  water  between  it  and  the  mainland  was 
filled  up,  and  the  whole  added  to  the  mainland  ;  the 
southern  coast  being  transferred  to  its  present  position, 
and  the  shoal  water,  with  its  mangrove  islands,  changed 
into  the  Everglades,  with  its  hummocks.  In  the  mean 
time,  however,  i.  e.,  while  the  present  southern  coast  was 
still  a  line  of  keys,  another  reef  was  formed  in  the  place 
of  the  present  line  of  keys,  and  the  former  have  therefore 
died.  This  new  reef  in  its  turn  was  converted  into  a  line 
of  keys,  which  will  eventually  coalesce  into  a  continuous 
line  of  land,  the  shoal  water  will  be  filled  up,  and  form 
another  Everglade,  with  its  hummocks,  and  the  coast- 
line be  transferred  to  the  present  line  of  keys.  But 
already  another  line  is  formed,  and  the  previous  line  is 
dead ;  already  the  process  of  key-formation  has  com- 
menced. We  can  not  doubt  that  eventually,  but  proba- 
bly only  after  many  thousands  of  years,  the  Peninsula  of 
Elorida  will  extend  even  to  the  present  living  reef.  Ear- 
ther  than  this  it  can  not  go,  for  the  deep  water  of  the 
Gulf  Stream  is  close  at  hand,  and  forms  its  impassable 
boundary. 

Earther  northward,  the  extent  of  the  coral  formation  is 
less  known,  but  it  has  been  found  on  the  eastern  coast  as 
far  as  St.  Augustine.  The  middle  and  western  part  of 
Elorida,  as  far  as  the  north  shore  of  the  Everglades,  is 
probably  older.  The  line  d  d  (Eig.  61,  A),  therefore, 
probably  marks  out  the  area  which  has  been  added  to 
Elorida   by  the   agencies   described.     The   area   already 


ORGANIC  AGENCIES,  113 

added  is  probably  not  less  than  12,000  to  15,000  square 
miles,  and  the  area  which  will  be  added  at  least  half  as 
much  more. 

Cooperation  of  Other  Agencies. 

We  have  seen  that  the  reefs  of  Florida  are  unique.  It 
seems  certain,  therefore,  that  they  were  formed  under 
unique  conditions.     The  things  to  be  accounted  for  are — 

1.  The  constant  growth  of  land  ;  and,  2.  The  formation 
of  barriers  where  there  was  no  subsidence. 

1.  The  constant  growth  of  land  southward  shows 
that  there  was  a  continual  extension  southward  of  the 
conditions  of  coral-growth,  i.  e.,  of  moderate  depth.  In 
other  words,  there  must  have  been  a  gradual  extension 
southward  of  the  submarine  bank,  on  the  edge  of  which 
the  corals  grew.  If  there  had  been  a  preexisting  bank, 
obviously  the  corals  would  have  grown  as  only  one  reef 
on  its  outer  edge  ;  the  formation  of  successive  reefs,  one 
beyond  the  other,  proves  that  the  shallow  bank  on  which 
they  grew  must  have  extended  successively  in  that  direc- 
tion. 

Thus  much  s-eems  certain,  but  the  cause  of  the  exten- 
sion is  more  uncertain.  It  is  probable,  however,  that  the 
bank  was  formed  and  extended  by  sedimentary  deposit  by 
the  Gulf  Stream.* 

Thus,  then,  the  extension  of  the  Peninsula  of  Florida 
in  recent  times  has  been  the  result  of  the  cooperation  of 
several  agencies :  1.  The  Gulf  Stream  built  up  from  deep- 
sea  bottom  a  bank,  and  extended  it  by  the  same  process. 

2.  The  corals  took  up  the  work  by  forming  successive  bar- 
*  At  one  time  the  sediments  were  supposed  to  be  mechanical  sedi- 
ments from  the  Gulf  rivers,  especially  the  Mississippi.  But  now  they 
are  believed  to  be  organic  sediments,  partly  brought  by  the  Gull 
Stream  from  other  coral  banks,  e.  g.,  the  Yucatan  bank,  but  mainly 
formed  in  place  by  the  growth  of  successive  generations  of  deep-sea 
shells  ;  the  Gulf  Stream  bringing  only  the  conditions  of  heat  and 
food  necesary  for  rapid  growth. 

Ls  CoNTE,  Geol.  8 


114  DYNAMICAL   OEOLOQY. 

rier-reefs  farther  and  farther  south  as  the  necessary  con- 
dition of  moderate  depth  extended.  3.  The  waves  then 
took  up  the  work  and  converted  tlie  line  of  reef  into  a 
line  of  keys,  and  finally  a  line  of  land  twelve  to  fifteen  feet 
high.  4.  The  shoal  waters  between  the  successive  lines  of 
keys  and  the  mainland  was  filled  up  by  coral  debris  carried 
inward  from  the  reef  and  keys,  and  southward  from  the 
previously  formed  land,  and  the  mainland  was  thus  ex- 
tended to  the  keys. 

2.  Barrier-reefs  without  subsidence  may  be  ac- 
counted for  thus  :  From  the  manner  in  which,  by  this 
view,  the  bases  of  the  coral  reefs  were  formed,  viz.,  by 
sedimentation,  there  must  always  have  existed  a  very  soft, 
shallow  sea-bottom.  Along  such  a  shore-line  ^fringing 
reef  could  not  form,  because  the  chafing  waves  stir  up  the 
mud.  But  at  a  distance  from  shore,  where  the  water  is  a 
hundred  feet  deep,  and  the  waves  no  longer  touch  the 
bottom,  a  line  of  reef  would  form,  limited  on  the  one 
side  by  the  muddiness  and  on  the  other  by  the  increasing 
depth  of  the  water.  This  would  be  in  form  a  barrier- 
reef,  but  wholly  different  in  significance  from  those  of 
the  Pacific. 

Shell  Limestone. 

Lime  is  constantly  carried  to  the  sea  by  rivers,  and  yet 
is  the  sea-water  not  saturated.  This  is  because  the  lime 
in  sea-water  is  constantly  being  drafted  upon  by  animals 
to  form  their  shells  and  skeletons.  These  remain  after 
their  death,  accumulate  as  lime-deposits,  and  harden  into 
limestone.  We  have  already  spoken  of  coral  limestone, 
but  other  animals  besides  corals  form  limestones,  and 
some  make  other  kinds  of  deposits  besides  lime.  Besides 
corals,  the  most  important  are  shell-deposits.  We  shall 
treat  these  under  two  heads,  Molluscous  Shells  and  Micro- 
scopic Shells.  And  here  we  would  again  invite  the  per- 
sonal observation  of  the  pupil. 


ORGANIC  AGENCIES. 


116 


1.  Molluscous  Shells. — These  inhabit  mostly  shallow 
water,  and  therefore  accumulate  mostly  along  shore-lines, 
and  may  be  observed  by  all  who  keep  their  mental  eyes 
open.  Each  generation  takes  lime  from  sea-water,  and 
leaves  it  as  shell  on  the  bottom.  These,  therefore^  accu- 
mulate until  deposits  of  enormous  thickness  and  extent 
are  often  formed.  Sometimes  the  accumulated  mass  may 
consist  of  one  species,  as  in  oyster-banks  ;  sometimes  of 
many  species.  The  deposit  may  be  purely  shelly,  or  shell 
mixed  with  mud,  or  mud  with  a  few  imbedded  shells. 
Again,  on  exposed  shore-lines  the  shells  will  be  broken  or 
even  comminuted,  and  on  quiet  shore-lines,  as  in  bays  or 
harbors,  they  will  be  perfect.  These  accumulations  are 
gradually  hardened  into  limestone  (Fig.  62). 


Fig.  62.— Modern  shell  limestone.    (After  Scott.) 


Application. — Now,  all  these  different  kinds  of  lime- 
stone or  shell  rock  are  found  far  away  from  present  seas 
and  high  up  in  the  mountains.  "We  are  thus  often  able  to 
trace  out  the  shore-lines  of  previous  geological  times,  and 
determine  not  only  the  species  which  then  lived,  but  also 
the  conditions  under  which  they  lived. 

2.  Microscopic    Shells. — These    are    some   of   vege- 


116 


DYNAMICAL   GEOLOGY. 


table,  some  of  animal  origin  ;  some  fresh  water,  some  ma- 
rine ;  some  composed  of  silica,  some  of  lime  carbonate. 
The  two  most  important  kinds  are  silicious  fresh-water 
deposits  of  vegetable  origin,  and  lime  carbonate,  deep-sea 
deposits  of  animal  origin. 

Fresh-Water  Deposits.— It  is  well  known  that  still 
waters  swarm  with  microscopic  unicelled  plants.     Most  ol 


Pig.  63.— a,  diatoma  vulgare  :  a.  side  view  of  friistnle  ;  6,  fnistule  dividing  itself. 
B,  grammatophora  serpentina  :  a,  front  and  side  view  ;  6,  front  and  end  view  oJ 
dividing  frustule. 


these  have  no  shells  and  leave  no  deposit.  But  one  kind 
— the  diatoms — form  shells  of  silica.  Generation  after 
generation  of  these  leave  their  shells  until  deposits  of  great 
thickness  and  extent  are  formed.  In  any  clear  and 
sluggish  stream,  if  we  examine  with  the  microscope  the 
slime  on  the  stones  at  the  bottom,  we  shall  find  living 
diatoms.  These  are  carried  by  freshets  into  ponds,  lakes, 
or  seas  into  which  the  streams  empty.  Usually  the  mud 
carried  with  them  is  so  abundant  that  they  will  not  be 
detectable  in  the  deposit  thus  formed.  But  in  large, 
deep,  clear  lakes,  like  Lake  Tahoe,  beyond  the  reach  of 
sedimentary  deposit,  the  deep  bottom-ooze  is  found  to  be 
composed  wholly  of  the  accumulated  shells  of  diatoms. 
Also  in  the  hot  springs  of  California  and  in  the  pools 
formed  by  the  accumulation  of  these  waters^  diatoms  are 


ORGANIC  AGENCIES.  117 

very  abundant,  and  deposits  of  these  shells  are  formed 
comparatively  rapidly. 

Application. — In  many  countries,  and  nowhere  more 
abundantly  than  in  California,  is  found  a  soft,  white,  very 
light  and  friable  earth,  often  many  feet  in  thickness  and 
many  square  miles  in  extent,  which,  under  the  microscope, 
is  seen  to  consist  wholly  of  shells — some  perfect,  some 
broken — of  diatoms.  It  is  only  by  the  study  of  deposits 
now  forming  that  we  may  hope  to  understand  the  condi- 
tions under  which  these  remarkable  deposits  were  formed. 

Deep-Sea  Deposits.  —  Many  deep-sea  explorations 
•have  been  recently  undertaken  by  the  governments  of 
Europe  and  the  United  States.  From  these  we  learn 
that  the  deep-sea  ooze  is  almost  everywhere  a  fine  white 
mud,  which  dried  looks  like  chalk,  and  under  the  micro- 
scope is  seen  to  be  mainly  composed  of  the  carbonate-of- 
lime  shells — some  perfect, 
more  broken,  most  of  all 
comminuted — of  Forami- 
nifera  (a  low  form  of  ani- 
mals). The  most  abun- 
dant form  is  Olobigerina 
(Fig.  64),  and  therefore 
this  ooze  is  often  called  glo- 

iigerifia  ooze.    Among  thQSe         f.^.  64.-Foraminiferal  ooze,     x  13. 
are  scattered  silicioUS  shells  (Agasslz  after  Murray  and  Eenard.) 

of  diatoms  and  several  other 

kinds  of  shells,  animal  and  vegetable.  All  of  these  prob- 
ably live  at  the  surface,  and  on  their  death  drop  to  the  bot- 
tom. So  that  we  may  imagine  a  continual  drizzle  of  these 
shells  falling  to  the  bottom.  These  deposits  are  certainly 
of  enormous  extent,  and  probably  of  great  thickness. 

Application. — There  is  one  geological  stratum  which 
bears  a  striking  resemblance  to  this  deep-sea  ooze,  viz., 
the  chalk  of  England,  France,  the  interior  of  Europe,  and 
our  own  western  plains.     The  origin  of  this  very  peculiar 


118  DYNAMICAL   GEOLOGY. 

stratum  will  hereafter  be  discussed  in  the  light  of  these 
facts. 


Sectioi^  IV. — Geographical  Distribution-  of  Species. 

No  one  can  go  to  a  foreign  country,  or  even  a  distant 
part  of  our  own  country,  as,  for  example,  from  the  eastern 
to  the  western  coast,  without  being  struck  with  the  great 
difference  in  the  native  animals  and  plants.  If  such  a  one 
has  been  trained  to  observe,  he  will  see  that  nearly  all  the 
species  are  entirely  different.  As  a  broad,  general  fact, 
every  country  has  its  own  native  species,  differing  more 
or  less  conspicuously  from  those  of  other  countries.  The 
laws  of  this  distribution  and  its  causes  have  recently 
attracted  much  attention,  and  are  a  subject  of  very  great 
interest.     We  can  only  give  the  briefest  outline. 

Faunas  and  Floras. — We  shall  hereafter  frequently 
use  the  terms  fauna  and  flora,  and  must  therefore  define, 
them.  The  whole  group  of  animals  inhabiting  one  place 
is  called  \i^  fauna,  and  of  plants  its  flora.  Thus,  we  may 
speak  of  the  fauna  and  flora  of  New  York,  or  Illinois,  or 
Oregon.  But  science  cares  nothing  for  such  arbitrary 
limits — it  deals  only  with  natural  boundaries.  A  natural 
fauna  or  flora  is  a  natural  group  of  animals  or  plants  in 
one  place,  differing  conspicuously  from  other  groups  in 
other  places,  and  separated  from  them  by  natural  bound- 
aries, geographical  or  climatic.  Among  the  climatic  con- 
ditions limiting  faunas  and  floras,  perhaps  the  most 
important  is  temperature,  and  we  shall  therefore  speak  of 
this  first.  Again,  plants,  being  fixed  to  the  soil,  are 
more  strictly  limited  than  animals,  and  we  shall  there- 
fore illustrate  the  laws  of  distribution  first  by  them. 
Again,  temperature  conditions  change  in  elevation  above 
the  surface,  and  in  latitude.     We  take  the  former  first. 

Botanical  Temperature  Keg-ions  in  Elevation. — 
For  this  we  take  a  high  mountain,  near  the  seashore  in 


OBGAIv'IC  AGEJSCIES. 


119 


tropical  regions,  because  we  find  there  all  the  regions 
(Fig.  Go).  In  going  up  such  a  mountain,  from  sea-level, 
1 1,  we  pass  through,  —  1 .  A  region  of  palms,  so  called 
because  of  the  abundance  of  palms  and  palm-like  forms, 


Fig.  65. 

such  as  bananas,  tree-ferns,  etc.  ;  2.  A  region  of  hard- 
wood, or  ordinary  foliferous  trees  ;  3.  A  region  in  which 
pines  and  pine-like  trees  predominate  ;  4.  A  treeless 
region,  in  which  are  only  shrubs,  herbs,  and  grasses  ;  and 
5.  A  plantless  region,  or  region  of  perpetual  snow. 
These  regions,  although  we  have  separated  them  by  lines, 
of  course  graduate  insensibly  into  each  other.  The  sec- 
ond region  may  often  be  subdivided  into  a  region  of 
evergreens  and  a  region  of  deciduous  hard-woods. 

Botanical  Temperature  Re- 
^ons  in  Latitude. — Now,  since 
the  above  regions  are  determined 
wholly  by  temperature,  and  since 
a  similar  decrease  of  temperature 
is  found  in  going  from  the  equator 
to  the  poles,  we  ought  to  expect 
similar  regions  in  latitude.  And 
such  we  find.  In  going  from  the 
equator  to  the  poles  we  find — 1.  A 
region  of  palms,  corresponding  to 
the  tropic  zone  ;  2.  A  region  of  hard-wood  trees,  corre- 
sponding to  the  temperate  zone  ;  3.  A  region  of  pines  and 


Pig.  G6. 


120  DYNAMICAL  GEOLOGY. 

vine-like  trees  and  birches,  corresponding  to  suharctit 

and  arctic  zones  ;  4.  A  circumpolar  region  of  shrubs  and 
grasses ;  and,  5.  Perhaps  a  plantless  or  nearly  plantless 
region  at  the  pole  of  cold.  Here,  again.  No.  2  may  be 
subdivided  into  a  ^(;«r»^ -temperate  region  of  evergreens, 
and  a  cooZ-temperate  region  of  deciduous  trees.  Here, 
again,  also  the  regions  graduate  insensibly  into  each 
other. 

We  have  been  speaking  thus  far  of  sea-level  or  near 
sea-level.  Of  course,  if  a  mountain  in  any  latitude  rises 
to  perpetual  snow,  we  will  have  on  its  sides  all  the  tern- 
perature  regions,  except  those  south  of  it.  Thus,  in 
ascending  the  Sierra  Nevada  we  have  a  temperate  region 
{No.  2)  at  base,  a  subarctic  region  (No.  3)  half-way  up, 
and  a  circumpolar  region  (No.  4)  at  the  summits. 

Completer  Definition  of  Regions. — 1.  All  organic 
forms  will  spread  in  all  directions  as  far  as  physical  con- 
ditions and  the  struggle  for  life  with  other  species  will 
allow.  The  area  over  which  they  thus  spread  may  be 
called  their  *^  range.''  Now,  the  range  of  one  species  may 
be  much  greater  than  that  of  another,  because  more 
hardy  ;  but  the  range  of  a  species  is  always  more  restricted 
than  its  genus,  for  when  the  species  can  go  no  farther, 
another  species  of  the  same  genus  will  continue  the  genus. 
For  the  same  reason  the  range  of  a  family  is  greater  than 
that  of  its  genera,  etc.  Thus,  for  example,  in  going  up 
the  Sierra  we  find  the  range  of  pines  extend  from  2,000 
to  10,000  feet  above  sea-level,  but  the  genus  is  represented 
by  a  succession  of  species  of  much  more  restricted  ranges. 
We  find,  first,  the  nut-^me.  (Finus  Sabiniana),  then  the 
yellow-ipine  (P.  ponder osa),  then  the  sugar-ipme  [P.  Lam- 
bertiana),  then  the  tama7'aclc-^m.Q  {P.  contorfa),  and, 
last,  the  mountain-^uiQ  {P.  flexilis).  Hereafter  we  shall 
speak  mostly  of  species. 

2.  We  have  said  that  the  several  temperature  regions 
graduate  insensibly  into  each  other.     We  will  now  explain 


ORGANIC  AGENCIES.  121 

in  what  sense  this  is  true.  Species,  then,  come  in  grad- 
ually on  the  borders  of  their  range,  reach  their  highest 
development  in  number  and  vigor  about  the  middle,  and 
pass  out  gradually  in  number  and  vigor  on  the  other 
border,  other  species  taking  their  place,  and  the  two 
ranges  overlapping  on  their  borders.     Thus,  in  Fig.  67, 


\ 


Fig.  67. 


a  a'  is  the  north  and  south  range  of  species  A,  and  h  h'  of 
species  B — the  height  of  the  curve  the  number  and  vigor 
of  the  individuals,  and  I  a'  the  overlap  of  ranges. 

3.  But  in  specific  character  there  is  no  such  gradual  pas- 
sage of  one  species  into  another — no  evidence  of  trans- 
mutation of  one  species  into  another,  nor  of  derivation  of 
one  species  from  another.  From  this  point  of  view  spe- 
cies seem  to  come  in  at  once  in  full  perfection,  remain 
substantially  unchanged  throughout  their  ranges,  and 
pass  out  at  once  on  the  other  border,  other  species  taking 
their  place  as  if  by  substitution,  not  transmutation.  It  is 
as  if  each  species  originated,  no  matter  how,  somewhere 
in  the  region  where  we  find  them,  and  then  spread  in  all 
directions  as  far  as  physical  conditions  and  struggle  with 
other  species  would  allow. 

We  can  best  make  this  plain  by  illustrations  :  The 
sweet-gum  or  liquidambar-tree  extends  from  the  borders 
of  Florida  to  the  banks  of  the  Ohio.  It  is  most  abundant 
and  vigorous,  indeed,  in  the  middle  regions,  and  dying 
out  on  the  borders,  where  it  is  replaced  by  other  species  : 
but  is  everywhere  the  same  species,  unmistakable  by  its 
five-starred  leaf,  winged  bark,  spinous  burr,  and  fragrant 
gum.  Again,  the  Eed-wood  {Sequoia)  ranges  from  south- 
ern California  to  the  borders  of  Oregon.     It  may  be  most 


122  DYNAMICAL   OEOLOGY. 

vigorous  in  the  middle  region — it  may  decrease  in  vigor 
and  number  on  its  borders ;  but  in  all  specific  characters, 
wood,  bark,  leaf,  and  burr,  it  is  the  same  throughout. 
The  study  of  species,  as  they  now  are,  would  probably  not 
suggest,  certainly  could  not  prove,  the  theory  of  their 
origin  by  derivation  or  transmutation. 

4.  Temperature  regions  shade  into  each  other.  But 
this  is  so  only  where  no  barriers  exist.  If  there  be  barriers, 
such  as  an  east  and  west  mountain-chain,  or  sea,  or  desert, 
then  on  the  two  sides  of  the  barrier  the  species  will  be 
very  distinct  and  without  gradation  by  overlapping. 
Thus,  north  and  south  of  tlie  Sahara,  and  north  and 
south  of  the  Himalayas,  there  is  a  marked  and,  as  it  were, 
a  sudden  change  of  species.  It  is,  again,  as  if  the  species 
originated  each  in  its  own  area  and  spread,  but  were  pre- 
vented from  mingling  and  overlapping  on  their  borders 
by  the  barrier. 

5.  Again,  although  there  are  similar  temperature  re- 
gions on  tropical  mountains  and  in  high  latitudes — and 
these  latter  are  also  repeated  north  and  south  of  the 
equator — yet  the  species  are  always  different  in  the  three 
cases.  This  is  because  the  torrid  zone  is  a  barrier  pre- 
venting migration.  It  is,  again,  as  if  species  originated 
each  in  its  own  place,  but  were  prevented  from  reaching 
similar  temperature  regions  elsewhere  by  the  existence  of 
impassable  barriers. 

Zoological  Temperature  Regions. — Animal  species 
are  limited  by  temperature,  like  plants,  and  therefore  also 
exist  in  temperature  zones  ;  but  they  can  not  be  arranged 
in  the  same  simple  way,  evident  even  to  the  popular  eye 
■ — i.  e.,  great  classes  corresponding  to  great  zones.  It  is 
true  that,  if  we  compare  extremes,  viz.,  polar  with  tropi- 
cal regions,  we  find  a  conspicuous  contrast  determined  by 
temperature,  certain  great  families  being  characteristic 
of  each — as,  for  example,  among  mammals,  the  great 
pachyderms,  the  elephant,  rhinoceros,  hippopotamus,  and 


ORGANIC  AGENCIES,  123 

the  great  cats,  lions,  tigers,  jaguars  ;  among  birds,  tou- 
cans, parrots,  trogons,  ostriches  ;  among  reptiles,  croco- 
dilians  and  pythons  ;  and  among  corals,  the  reef-builders, 
characterizing  the  tropics ;  while  the  musk-ox,  white 
bear,  seals,  walrus,  ducks  and  geese,  characterize  the 
polar  regions — yet  we  can  not  make  a  zonal  arrangement 
of  families  as  easily  as  we  can  with  plants.  But,  confin- 
ing our  attention  to  species  or  even  genera,  animal  forms 
are  subject  to  the  same  laws  as  those  of  plants  :  1.  All 
animal  species  are  limited  in  range  ;  2.  The  range  of 
species  is  more  limited  than  that  of  genera,  and  of  genera 
than  that  of  families,  etc.;  3.  Contiguous  ranges  grad- 
uate into  each  other  by  overlapping,  the  species  inter- 
mingling and  coexisting  on  the  margins ;  4.  Each  species 
reaches  a  maximum  of  number  and  vigor  in  middle  regions 
and  dies  out  on  the  borders ;  5.  But  in  specific  character 
they  seem  to  remain  substantially  the  same  throughout 
their  range,  and  do  not  change  or  transmute  into  other 
species  on  the  borders  ;  6.  Physical  conditions  may  limit 
their  range,  but  do  not  seem  to  change  them  into  other 
species,  though  varieties  may  be  formed  in  this  way  ; 
7.  Here,  again,  it  is  as  if  species  originated,  no  matter 
how,  in  the  places  where  we  find  them,  and  have  spread 
in  all  directions  as  far  as  physical  conditions  and  struggle 
with  other  species  would  allow.  All  that  we  shall  say 
hereafter  will  apply  equally  to  animals  and  plants. 

Continental  Faunas  and  Floras. — If  there  were  no 
barriers  to  the  spread  of  species  around  the  earth  on  the 
same  zone,  there  can  be  no  doubt  that  they  would  thus 
spread,  and  faunas  and  floras  would  be  arranged  in  a 
series  of  temperature  zones  from  the  equator  to  the  poles, 
containing  the  same  species  all  around.  But  the  oceans 
are  impassable  barriers  between  the  continents,  and  there- 
fore the  faunas  and  floras  of  different  continents  are  sub- 
stantially different.  It  is,  again,  as  t/they  originated  on 
the  continents  where  we  find  them,  and  have  been  pre* 


124  DYNAMICAL   GEOLOGY. 

vented  from  spreading  and  intermingling  by  the  impassa- 
ble barrier  of  the  ocean.  Even  apparent  exceptions, 
when  examined,  confirm  the  law,  as  we  now  proceed  to 
show. 

Fig.  68  is  a  nortli-polar  view  of  the  earth,  and  1,  2,  3, 
4,  5,  are  the  temperature  zones  so  often  referred  to.     Now. 


Fig.  68 

commencing  with  Nos.  4  and  5,  the  species  in  the  Eastern 
and  Western  Continents  are  substantially  the  same,  for 
the  lands  of  the  two  continents  approach  each  other  in 
these  zones  so  nearly  that  they  may  be  considered  as  one. 
There  is  no  barrier  to  the  spread  of  species  all  around  the 
pole.  But  when  we  come  to  No.  3,  and  still  more  to 
No.  2,  the  difference  of  species  is  almost  complete,  and 
many  genera  are  also  different — and  that,  not  because  the 
physical  conditions  are  unsuitable  ;  for  European  species 
introduced  in  our  country  do  so  well  that  they  often  kill 
out  our  own  native  species.  Nearly  all  useful  and  nox- 
ious species  have  been  thus  introduced.  They  were  not 
here  when  America  was  discovered  only  because  they 
could  not  get  here. 


» 


OROANIG  AGENCIES.  125 

We  said  the  difference  is  almost  complete.  There  are, 
therefore,  some  exceptions,  but  these  only  confirm  the 
principles  on  which  the  rule  is  founded.  They  are  of  three 
kinds  :  1.  Hardy  or  widely  migrating  species.  Some 
hardy  species  range  through  No.  3  into  No.  4,  and  these 
may  pass  over  from  continent  to  continent.  Some  birds, 
like  the  Canada  goose  and  mallard  duck,  migrate  in  sum- 
mer to  No.  4,  and  thence  in  winter  southward  in  both 
continents.  2.  Introdticed  species,  which  have  become 
wild.  3.  Alpine  species,  mostly  of  insects  and  plants.  It 
is  a  curious  fact  that  species  of  plants  and  insects, 
isolated  on  the  tops  of  high  mountains  near  the  snow- 
line, are  similar  to  each  other  on  the  two  continents, 
and  also  similar  to  Arctic  species.  This  latter  fact  gives 
the  key  to  the  explanation.  The  geological  epoch  imme- 
diately preceding  the  present  (glacial  epoch)  was  charac- 
terized by  extension  of  Arctic  conditions  southward  even 
to  the  shores  of  the  Mediterranean  and  the  Gulf  of  Mex- 
ico. At  that  time,  therefore,  Arctic  species  occupied  all 
Europe  and  the  United  States.  As  the  cold  abated,  Arc- 
tic species  mostly  went  northward  to  their  present  home 
in  the  Arctic  zone.  But  some  followed  the  receding  Arc- 
tic conditions  upward  to  the  tops  of  mountains,  and  were 
left  stranded  there,  both  in  Europe  and  this  country. 

In  No.  1  the  species  on  the  two  continents  are  still 
more  markedly  different,  the  difference  extending  even 
to  families  and  in  some  instances  to  orders.  Thus,  for 
example,  among  plants,  the  cactus  order  is  confined  to 
America.  Among  animals,  the  great  pachyderms,  e.  g., 
elephants,  rhinoceroses,  hippopotamuses,  also  the  camels, 
horses,  and  tailless  monkeys,  are  confined  to  the  Old 
World,  while  the  sloths,  the  armadillos,  the  prehensile- 
tailed  monkeys,  the  whole  family  of  humming-birds  (of 
which  there  are  over  four  hundred  species),  and  .the 
family  of  toucans,  are  confined  to  the  New. 

South  of  the  equator  the  continents  do  not  again  ap- 


126  DYNAMICAL   GEOLOGY. 

proach,  and  therefore  the  fauna  of  Africa  and  South 
America  remain  very  different  even  to  Cape  Colony  and 
Fuegia. 

Subdivisions. — Continental  faunas  and  floras  are  again 
subdivided  in  longitude,  more  or  less  completely,  by  bar- 
riers in  the  form  of  north  and  south  mountain-chains. 
Thus  the  fauna  and  flora  of  the  United  States  are  sub- 
divided by  the  Eocky  Mountain  and  Appalachian  chain 
into  three  sub-faunas  and  floras,  an  Atlantic  slope,  a 
Mississippi  basin,  and  a  Pacific  slope.  The  difference 
between  these  is  strictly  in  proportion  to  the  impassahle- 
ness  of  the  barriers.  Thus,  between  the  Atlantic  slope 
and  the  Mississippi  basin  the  difference  is  very  small, 
because  the  Appalachian  chain  is  low  and  may  be  over- 
passed ;  but  the  Pacific  slope  fauna  and  flora  are  almost 
wholly  peculiar.  Almost  the  only  exceptions  are  strong- 
winged  birds,  like  the  turtle-dove,  the  turkey-vulture,  the 
large  blue  heron,  etc.  In  many  cases  the  species  are  very 
similar  and  yet  different.  The  meadow-lark  and  the  yel- 
low-hammer are  examples.  Similarly  the  Ural  Mountains 
separate  a  European  from  a  northern  Asiatic  fauna  and 
flora.  These  subdivisions  are  perhaps  more  marked  in 
case  of  plants  than  animals.  The  spread  of  plants  is  pas- 
sive (dispersal),  the  spread  of  higher  animals  also  by 
migration. 

Special  Cases. — Isolated  islands,  and  in  proportion  to 
the  degree  of  their  isolation,  have  peculiar  species.  We 
shall  mention  only  a  few  cases  as  examples  of  a  general 
law. 

Australia  is  undoubtedly  the  most  striking  case  of  all. 
The  trees  of  this  isolated  continent  are  so  different  from 
those  of  the  rest  of  the  world  that  the  whole  aspect  of 
field  and  forest  is  peculiar  and  strange.  The  animals  are 
not  only  all  different  in  species,  but  the  genera  and  fami- 
lies and  even  many  orders  are  peculiar.  Of  two  hundred 
species  of  mammals,  nearly  all  belong  to  a  distinct  sub- 


ORGANIC  AGENCIES,  127 

class  (non-placentals),  including  kangaroos,  opossums, 
ornithorhynchus,  etc.,  which,  with  the  exception  of  a  few 
species  of  opossums,  are  found  only  in  Australia  and  the 
island  appendages  of  that  continent.  Madagascar  is  sep- 
arated by  a  deep  sea  from  Africa,  and  we  therefore  find 
the  organic  forms  entirely  different  from  those  of  the 
neighboring  continent,  or  of  any  other  part  of  the  world. 
It  is  especially  characterized  by  the  great  number  of 
lemurs.  On  the  Galapagos  (a  small  group  of  islands  off 
the  western  coast  of  South  America,  but  separated  by  a 
deep  sea)  the  animals  and  plants  are  all  peculiar.  Eeptiles 
of  strange  aspect  abound,  but  no  mammals  (except,  per- 
haps, one  species  of  mouse)  are  known. 

Thus  we  see  that  species  are  limited  north  and  south 
by  temperature,  and  in  every  direction  by  physical  bar- 
riers. If,  now,  we  add  peculiar  soil  and  climates  (as  in 
Utah,  Arizona,  etc.),  which,  of  course,  control  vegetation 
and,  therefore,  animal  life,  it  is  easy  to  see  that  all  these 
limiting  causes  produce  groups  of  species  confined  within 
certain  areas,  and  differing  from  other  groups,  sometimes 
overlapping  and  sometimes  trenchantly  separated. 

Element  of  Time. — We  have  said  that  faunas  and 
floras  differ  in  proportion  to  the  impassableness  of  the  bar- 
riers between — i.  e.,  the  height  and  breadth  of  the  moun- 
tain-chains, the  extent  of  deserts,  and  the  width  and  depth 
of  seas,  etc.  But  there  is  still  another  element  of  the 
■greatest  importance,  viz.,  the  length  of  time  elapsed  since 
the  barrier  was  set  up.  This  element  of  time  connects 
geographical  faunas  and  floras  with  geological  changes, 
and  thus  geographical  distribution  of  species  becomes  the 
key  to  the  most  recent  of  these  changes.  If  we  suppose 
species  to  undergo  very  slow  changes,  then  the  longer 
faunas  are  separated  the  greater  becomes  their  difference. 
The  full  discussion  of  this  important  point  requires  a 
knowledge  of  the  general  laws  of  evolution,  which  we  are 
not  yet  prepared  to  take  up. 


128 


DYNAMICAL  OEOLOOY. 


Primary  Regions  and  Provinces. — Taking  all  the 
causes  into  account,  the  whole  land-surface  has  been 
divided  by  Mr.  Wallace  into  six  faunal  regions — viz.  : 


Fig.  69. 


1.  Nearctic,  including  all  North  America  exclusive  of  Cen- 
tral America.  2.  Neotropic,  including  Central  and  South 
America.  3.  Palcearctie,  including  Europe,  Africa  north 
of  the  Sahara,  and  Asia  north  of  the  Himalayas.  4.  Afri- 
can, including  Africa  south  of  the  Sahara  and  Madagascar. 
5.  Oriental,  including  Asia  south  of  the  Himalayas  and 
all  the  adjacent  islands.  6.  Australian,  including  Aus- 
tralia, New  Zealand,  New  Guinea,  and  the  South  Sea 
Islands. 

These  primary  regions  are  subdivided  into  provinces, 
and  these  into  sub-provinces,  according  to  the  principles 
already  explained.  We  will  illustrate  by  only  one  example. 
The  Nearctic  region  is  divided  into  four  provinces  :  1. 


OROAmc  AGENCIJSS.  1^9 

Alleghanian ;  2.  Canadian  or  boreal ;  3.  Rochy  Mountain ; 
•  and,  4.  Calif ornian.     The  limits  of  these  are  shown  in 
Fig.  69. 

Marine  Faunas. 

Conditions  are  far  less  diverse  in  the  sea  than  on  land, 
and  the  limitation  of  fauna  is  less  marked,  but  the  same 
laws  hold. 

Temperature  Regions  in  Latitude. — Fauna  are  here 
also  arranged  in  zones  determined  by  temperature.  In  a 
north  and  south  coast-line,  where  the  temperature  changes 
gradually,  the  fauna  will  also  change  gradually  by  the 
substitution  of  one  species  for  another;  but  if  for  any 
cause  there  is  a  more  sudden  change  of  oceanic  tempera- 
ture, there  will  be  a  correspondingly  rapid  change  of 
fauna.  For  example,  on  our  Atlantic  coast, '  the  Gulf 
Stream  hugs  the  southern  coast  as  far  as  Cape  Hatteras 
(Fig.  69,  a)y  and  then  turns  away  and  runs  at  a  greater 
distance.  This  makes  a  great  change  of  temperature  at 
this  point.  Again,  the  Arctic  current,  c,  coming  out  of 
Baffin^s  Bay,  hugs  the  coast  of  New  England  as  far  as 
Cape  Cod,  J,  and  then  goes  down.  Thus  Arctic  condi- 
tions prevail  in  coast  waters  to  this  point.  Thus  there 
are  three  very  different  marine  faunas  along  the  coast  of 
the  United  States — viz.,  a  Southern,  a  Middle  State,  and 
a  Northern ;  and  these  change  somewhat  suddenly  at 
Capes  Hatteras  and  Cod. 

Distribution  in  Longitude. — By  far  the  larger  num- 
ber of  marine  species  inhabit  along  shore.  For  these  the 
deep  sea  is  a  barrier  no  less  impassable  than  the  land. 
Therefore,  the  species  inhabiting  the  two  shores  of  an 
ocean  like  the  Atlantic  are  as  completely  different  as  those 
inhabiting  along  the  two  coasts  of  a  continent,  as  America. 

Special  Cases. — There  are  many  species  which  live 
in  the  open  sea  and  form  a  distinctive  Pelagic  fauna. 
Again,  there  are  others  which  are  conditioned  by  d^Dth 

Le  Conte,  Geol.  9 


130  DYNAMICAL  QEOLOOY, 

and  character  of  bottom.     The  most  remarkable  of  these 
are  those  inhabiting   deep-sea  bottom,   and  forming  an  • 
abyssal  fauna.     Again,  about  the  shores  of  isolated  islands, 
as  Madagascar  and  Australia,  the  marine  fauna  are  as 
peculiar  as  the  land  fauna. 

Origin  of  Geographical  Diversity, 

Until  recently  the  most  reasonable  view  seemed  to  be 
that  species  originated  where  we  find  them,  and  spread  in 
all  directions  as  far  as  they  could.  According  to  this 
view,  the  difference  between  faunas  ought  to  be  strictly 
in  proportion  to  the  impassableness  of  the  barriers  be- 
tween. This  is  largely  true,  but  does  not  account  for  all 
the  phenomena.  There  is  another  element  of  equal  im- 
portance, viz.,  the  time  during  which  the  harrier  has 
existed.  It  is  probable  that  faunas  and  floras  are  subject 
to  slow,  progressive  changes,  taking  different  directions 
in  different  places.  If  there  be  no  barriers,  spreading  by 
dispersal  or  migration  prevents  extreme  diversity.  But 
if  a  barrier  be  at  any  time  set  up  by  geological  changes, 
then  diversity  commences,  and  increases  with  time. 
According  to  this  view,  the  Australian  fauna  is  so  peculiar 
because  this  continent  has  been  so  long  isolated  from  all 
others.  The  fauna  of  islands  off  the  coasts  of  continents 
are  often  very  similar  to  that  of  the  adjacent  mainland, 
because  they  have  only  recently  been  separated.  Thus, 
for  example,  the  fauna  and  flora  of  the  British  Isles 
differ  but  very  slightly  from  those  of  the  Continent, 
because,  as  we  now  know,  these  islands,  even  since  their 
inhabitation  by  man,  have  been  in  full  connection  with 
Europe.  The  divergence  has  commenced,  but  is  only 
varietal,  not  specific.  This  subject  will  be  taken  up 
again,  and  more  fully  explained  in  connection  with  glacial 
epoch,  p.  403. 


CHAPTER  IV. 

IGNEOUS    AGENCIES. 

All  the  agencies  which  we  have  thus  far  discussed 
tend  to  destroy  the  great  inequalities  of  the  earth-surface 
by  cutting  down  the  land  and  filling  up  the  seas.  They 
are  therefore  called  leveling  agencies.  If  they  alone  acted, 
they  would  eventually  bring  all  to  the  sea-level  and  inau- 
gurate a  universal  ocean.  These  agencies,  however,  are 
opposed  by  igneous  or  by  elevating  agencies,  which,  acting 
alone,  would  make  the  inequalities  much  greater  than  we 
now  find  them.  The  actual  amount  and  distribution  of 
land  are  the  result  of  the  state  of  balance  between  these 

ttwo  opposite  forces.  It  is  well  to  observe  that  the  leveling 
forces  are  derived  from  the  sun,  while  the  elevating  forces 
are  derived  from  the  interior  of  the  earth — being  in  fact 
connected  with  interior  heat.  It  becomes  necessary, 
therefore,  first  of  all  to  discuss  this  subject. 

Interior  Heat  of  the  Earth, 

The  surface  temperature  of  the  earth  varies  with  lati- 
tude, but  the  mean  is  about  60°.  At  any  place  the 
surface  temperature  varies  between  night  and  day  (daily 
variation),  and  between  summer  and  winter  (annual 
variation).  As  we  go  below  the  surface,  both  the  daily 
and  the  annual  variation  become  less  and  less,  and  finally 
disappear.  The  daily  variation  disappears  in  a  few  feet, 
fe  but  the  annual  variation  continues  and  disappears  in  our 
^     latitude  only  at  a  depth  of  sixty  or  more  feet.     Below 


133  DYNAMICAL   GEOLOGY. 

this  the  temperature  is  invariable.  The  upper  limit  of 
the  region  of  invariable  temperature  is  called  the  stratum 
of  invariable  temperature.  Its  depth  varies  with  latitude, 
being  nearest  the  surface  at  the  equator,  and  lying  deeper 
as  we  go  poleward. 

As  already  said,  below  this  stratum  the  temperature 
is  invariable,  but  it  increases  as  toe  go  deeper.  This 
important  fact  Las  been  proved  by  observations  in  mines 
and  artesian  wells.  It  is  true  everywhere,  but  the  rate 
of  increase  varies,  being  in  some  places  more  rapid  (1° 
in  thirty  feet),  in  some  less  rapid  (1°  in  ninety  feet). 
The  average  may  be  taken,  for  convenience,  at  1°  for 
every  fifty-three  feet,  or  100°  for  every  mile  of  depth. 

The  Interior  Condition  of  the  Earth. — Now  it  is 
easy  to  see  that  at  this  rate  the  melting  temperature  for 
most  rocks,  say  3,000°,  would  be  reached  at  a  depth  of 
about  thirty  miles.  Hence,  many  persons  have  rashly 
concluded  that  the  earth  is  essentially  an  incandescent, 
liquid  mass,  covered  with  a  comparatively  thin  shell  of 
thirty  miles.  This  would  correspond,  in  a  ball  of  two 
feet  diameter,  to  a  shell  of  less  than  one  tenth  inch  thick. 
On  this  view,  volcanoes  are  supposed  to  be  openings  into 
this  general  interior  liquid. 

A  little  reflection,  however,  suffices  to  show  that  this 
condition  of  the  interior  is  improbable.  It  is  almost  cer- 
tain, in  the  first  place,  that  the  rate  of  increase  is  not 
uniform,  but  decreases,  and  therefore  that  the  temperature 
of  3,000°  would  be  found  only  at  a  much  greater  depth 
than  thirty  miles.  In  the  second  place,  3,000°  is  the 
fusing  point  under  atmospheric  pressure  ;  but  under  the 
enormous  pressure  of  thirty  to  fifty  miles  of  rock,  the  fus- 
ing point  would  probably  be  much  higher.  Taking  these 
two  things  into  account,  it  seems  certain  that,  if  there  be 
a  universal  interior  liquid  at  all,  the  solid  shell  is  much 
thicker  than  is  usually  supposed,  and  even  probable  that 
there  is  no  universal  interior  liquid  at  all — and  that  vol- 


I 


laJSTEOtrS  AGENCIES.  .  133 

canoes  are  openings  into  local  reservoirs,  not  into  a  uni- 
versal sea  of  liquid  matter. 

Eecently  there  lias  been  a  tendency  among  geologists 
to  accept  a  compromise  between  these  extremes.  It  is 
now  well  known  that  rocks,  under  the  combined  influence 
of  heat  and  water,  fuse  at  a  much  lower  temperature. 
This,  to  distinguish  it  from  true  dry  fusion,  is  called 
hydrothermal  fusion.  While  the  temperature  of  true 
fusion  is  not  less  than  3,000°,  that  of  hydrothermal  fusion 
is  only  600°  to  800°.  Now,  water  certainly  penetrates  the 
earth  to  great  depths.  Therefore  many  think  that  the 
general  constitution  of  the  earth  is  that  of  a  solid  nucleus 
and  a  solid  crust,  separated  by  a  sub-crust  layer  of  liquid 
or  semi-liquid  matter.  There  are  many  geological  phe- 
nomena that  are  best  explained  by  such  a  supposition. 

The  interior  heat  of  the  earth  is  the  source  of  all  igne- 
ous agencies.  It  shows  itself  on  the  surface  in  three 
principal  forms,  viz. :  1.  Volcanoes;  2.  Earthquakes;  3. 
Gradual  oscillations  of  the  crust. 

Section  I. — Volcanoes. 

Definition. — A  volcano  may  be  defined  as  a  conical 
mountain,  with  a  pit-shaped,  cup-shaped,  or  funnel- 
shaped  opening  atop,  from  which  are  ejected,  from  time 
to  time,  materials  of  various  kinds,  always  hot  and  often 
fused.  They  vary  in  size  from  inconspicuous  mounds  to 
mountains  many  thousand  feet  high. 

Volcanoes  may  be  active  or  extinct.  Those  which  have 
not  erupted  for  a  century  past  are  supposed  to  be  extinct. 
Yet,  so-called  extinct  volcanoes  sometimes  break  out 
again.  Until  the  great  eruption  which  destroyed  Hercu- 
laneum  and  Pompeii,  Vesuvius  was  supposed  to  be  an 
extinct  cone.  Since  that  time  it  has  been  very  active. 
Again,  in  some  rare  cases,  volcanic  eruptions  are  constant. 
Stromboli  and  Krlauea,  for  example,  are  in  feeble  erup- 


134  DYNAMICAL  GEOLOGY. 

tion  all  the  time.  But  most  volcanic  eruptions  are  peri- 
odic The  period  of  intermission  may  be  ten,  or  twenty, 
or  fifty,  or  one  hundred  years. 

Number,  Size,  and  Distribution. — Humboldt  enu- 
merates 225  volcanoes  as  known  to  have  erupted  in  the 
past  century.  The  number  now  known  is  doubtless  much 
greater.  In  size  they  vary  from  little  mounds  (monticles) 
to  Mount  Etna,  11,000  feet;  Mauna  Loa,  14,000  feet; 
and  Aconcagua,  23,000  feet.  In  this  last  case  the  whole 
height  is  not  due  to  volcanic  eruptions,  for  the  cone 
stands  on  a  mountain  plateau  many  thousand  feet  high  ; 
but  the  others  are  wholly  built  up  by  eruption.  The 
laws  of  distribution  may  be  briefly  stated  as  follows  :  1. 
Volcanoes  are  mostly  on  islands  in  the  midst  of  the  sea, 
or  on  the  margins  of  continents  bordering  the  sea. 
Only  a  very  few  have  been  found  at  a  distance  from  the 
sea.  The  Pacific  Ocean  is  the  greatest  theatre  of  volcanic 
activity.  Its  surface  is  dotted  over  with  volcanic  islands, 
and  its  margin  is  belted  about  with  a  fiery  girdle  of 
volcanic  vents.  2.  Volcanoes  occur  usually  in  lines,  as 
if  connected  with  a  crust  fissure,  or  else  in  groups,  as  if 
connected  with  a  subterranean  lake  of  fused  matter. 
The  most  remarkable  linear  series  of  volcanoes  is  that 
which,  commencing  in  the  volcanoes  of  Fuegia,  con- 
tinues, as  a  chain  of  active  vents,  along  the  Andes 
and  Mexican  Cordilleras  ;  then  along  the  Sierra  and 
Cascade,  as  the  recently  extinct  volcanoes  of  these 
chains  ;  then  along  the  Aleutian  Isles  and  Kamschatka ; 
then  through  the  Kurile  Isles  to  Japan  and  the  Philip- 
pines ;  then  with  more  uncertainty  to  New  Guinea,  N"ew 
Zealand,  the  Antarctic  Continent,  Deception  Island,  and 
back  again  to  Fuegia,  after  completing  the  circle  of  the 
globe.  The  ijiost  remarkable  groups  are  the  Javanese 
group,  the  Hawaiian  group,  the  Icelandic  and  the  Medi- 
terranean groups.  3.  Volcanoes  are  found  mostly  in 
strata  of  comparatively  recent  date,  and  the  retiring  of 


IGNEOUS  AGENCIES,  135 

the  sea  seems  in  many  cases  to  be  associated  with  their 
gradual  extinction.  The  recently  extinct  volcanoes  on 
the  east  side  of  the  Sierra  are  good  examples. 

Phenomena  of  an  Eruption. — In  some  cases,  as  in 
the  Hawaiian  volcanoes,  the  floor  of  the  crater,  hardened 
from  previous  eruption,  becomes  hot,  then  melts  ;  then 
the  melted  lava  rises  higher  and  higher,  until  it  overflows 
and  runs  down  the  slope  in  one  or  more  streams.  The 
volcanic  forces  being  thus  relieved,  the  melted  lava  again 
sinks  gradually  to  its  former  level,  and  hardens  into  a 
floor.  Thus  all  proceeds  with  but  little  commotion.  In 
other  cases,  as  in  the  Javanese  volcanoes,  premonitions  of 
coming  violence  are  observed  in  the  form  of  subterranean 
explosions  attended  with  shakings  of  the  earth ;  then, 
with  a  powerful  explosion,  the  floor  of  the  crater  is  broken 
up,  and  the  fragments  are  shot  with  violence,  high,  some- 
times miles  high,  in  the  air ;  then  cinders  and  ashes  and 
smohe  are  ejected  in  immense  volumes  ;  then  streams  of 
lava  are  outpoured,  perhaps  alternating  with  explosions 
of  gas  and  vapor,  ejecting  cinders  and  ashes.  The  ascend- 
ing vapors  are  condensed,  and  fall  as  deluges  of  rain, 
which,  with  ejected  ashes,  form  streams  of  mud.  In  all 
cases,  if  the  mountain  be  snow-capped,  the  melting  of 
the  snow  produces  floods,  which  are  often  among  the 
most  disastrous  features  of  the  eruption. 

Thus  there  are  two  extreme  types  of  eruptions,  the 
quiet  and  the  explosive.  In  the  one,  the  ejecta  are  mostly 
lavas;  in  the  other,  gases,  vapors,  ashes,  and  cinders. 
The  best  type  of  the  former  are  the  Hawaiian,  of  the 
latter  the  Javanese  volcanoes.  But  all  grades  between 
exist.  The  Icelandic  volcanoes  belong  more  nearly  to  the 
former  type,  the  Mediterranean  to  the  latter.  Among 
Mediterranean,  Etna  approaches  more  the  former,  and 
Vesuvius  the  latter. 

Quantity  of  Matter  i;jected. — In  the  great  eruption 
of  Tomboro,  in  the  Island  of  Sumbawa  (one  of  the  Jav- 


136  DYNAMICAL  GEOLOGY, 

anese  group),  in  1815,  the  explosions  are  said  to  have 
been  heard  in  Ceylon,  nine  hundred  miles  distant.  •  The 
quantity  of  smoke  and  ashes  was  so  great  that,  hanging 
in  the  air,  they  produced  absolute  darkness  for  many 
days,  and  falling,  covered  the  sea  over  an  area  of  one 
hundred  miles  radius.  It  has  been  estimated  that  the 
ashes  ejected  were  sufficient  to  cover  the  whole  of  Ger- 
many two  feet  deep,  and  if  piled  in  one  place  would  make 
a  mass  three  times  the  bulk  of  Mont  Blanc  (Herschel). 

Of  lava-eruptions,  perhaps  the  greatest  is  that  of  Reyk- 
janes  (Skaptar)  in  1783.  The  mass  outpoured  has  beejr 
estimated  as  twenty-one  cubic  miles  (Herschel).  These, 
however,  are  extreme  cases.  One  of  the  greatest  erup- 
tions of  Kilauea,  that  of  1840,  poured  out  a  lava-stream 
forty  miles  long,  which,  if  accumulated  in  one  place, 
would  cover  an  area  of  a  square  mile  eight  hundred  feet 
deep.  The  average  of  lava-flows,  however,  is  far  less. 
One  of  the  greatest  eruptions  of  Vesuvius  poured  out 
600,000,000  cubic  feet  of  lava.  This  would  cover  a 
square  mile  twenty-two  feet  deep,  or  would  make  a 
stream  seven  miles  long,  one  mile  wide,  and  three  feet 
thick. 

Monticles. — In  volcanoes  of  moderate  height,  eruptions 
usually  come  from  the  top  of  the  cone  or  principal  crater, 
but  in  very  lofty  volcanoes  the  pressure  necessary  to  raise 
lava  so  high  fissures  the  mountain  in  a  radiating  manner. 
These  fissures  are  filled  with  liquid  matter,  which,  on 
hardening,  form  radiating  dikes.  Eruptions  often  take 
place  through  these  fissures,  and  thus  form  subordinate 
craters  and  cones  about  the  main  cone  ;  these  are  called 
monticles.  About  six  hundred  such  monticles  dot  the 
surface  of  Mount  Etna,  some  of  which  are  seven  hundred 
feet  high  above  the  level  of  the  mountain-slope  on  which 
fchey  stand.  About  Mount  Shasta  (wliicli  is  a  recently 
extinct  volcano)  are  found  a  number  of  these  monticles. 

Nature  of  the  Materials  Eruptecl. — The   materials 


IGNEOUS  AGENCIES. 


137 


erupted  are — 1.  Rock-fragments.  2.  Lava.  3.  Cinders. 
4.  Sand.  5.  Ashes.  6.  Smoke.  7.  Gas.  The  rock-frag- 
ments are  formed  in  explosive  eruptions  by  the  breaking 
up  of  the  hardened  floor  of  the  crater,  and  require  no 
further  explanation. 

Lava. — This  term  is  applied  to  melted  rock,  or  to  the 
same  after  it  has  hardened  again.  The  degree  of  liquid- 
ity depends  partly  on  the  degree  of  heat  and  partly  on  the 
kind  of  fusion.  The  lava  of  Kilauea  is  as  liquid  as  honey. 
The  bursting  of  bubbles  on  the  surface  of  this  thin,  vis- 


FiG.  70.— Lava-tunnel,  and  "  spatter-cone  "  formed  by  escaping  steam,  Kilauea. 
(Photograph  by  Libbey.) 

cous  liquid  draws  it  out  into  hair-like  threads  like  spun- 
glass,  which  is  borne  by  the  winds  and  accumulated  in 
certain  parts  of  the  crater.  This  is  called  ''  Pele's  hair.'' 
Thin  lava  like  this,  when  it  first  issues  from  the  crater, 
runs  with  great  velocity,  twenty  to  twenty-five  miles  an 
hour  ;  but  as  it  cools  it  becomes  stiffer,  first  like  tar,  then 


138  DYNAMICAL   GEOLOGY. 

like  pitch,  and  therefore  runs  with  less  and  less  speed, 
until  it  becomes  rigid  and  stops.  Being  a  bad  conductor 
of  heat,  lava  cools  and  forms  a  crust  on  the  surface  while 
it  is  still  liquid  and  flowing  within.  The  liquid  finally 
flowing  out,  often  leaves  a  hollow  tube  or  gallery.  Again, 
since  all  lava  contains  more  or  less  of  gas  and  vapor,  the 
crust  is  a  sort  of  concreted  lava-foam.  This  vesicular, 
spongy  lava  is  called  scoria.  Sometimes,  in  very  stiffly 
viscous  lava,  the  vapor-bubbles  run  together  and  form 
huge  blisters,  which,  by  hardening,  form  caves.  Thus, 
nearly  all  lava-beds  are  full  of  galleries  and  caves.  It 
was  in  the  galleries  and  caves  which  honey-combed  the 
ancient  lava-flows  of  southern  Oregon  that  a  handful  of 
Modocs  defied  so  long  the  power  of  the  United  States 
Army. 

Again,  the  liquidity  of  lava,  and  its  appearance  after 
solidifying,  depend  much  upon  the  kind  of  fusion.  Lavas 
are  often  in  a  state  of  hydr other 7nal  fusion  (page  133), 
i.  e.,  half  fusion,  half  solution  in  superheated  water.  Such 
a  semi-fused  mass,  on  concreting,  makes  a  kind  of  earthy 
stone.  Sometimes,  in  fact,  the  ejecta  are  little  more  than 
hot  mud,  and  concrete  into  tufa. 

Cinders,  Sand,  and  Ashes  are  only  different  forms 
of  hardened  lava.  The  liquid  lava,  before  ejection,  may 
be  so  largely  mingled  with  gas  and  vapors  that  it  is  liter- 
ally a  roch-foam.  Masses  of  this  rock -foam,  ejected  with 
violence  into  the  air,  cool  and  fall  as  cinders.  Often 
the  greater  part  of  the  ejections  is  of  this  kind,  and  thus 
are  formed  cinder-cones.  Sometimes  the  violence  of  the 
explosions  is  so  great  as  to  break  up  the  liquid  mass  into 
rock-spray.  This  falls  again  as  sand  or  ashes,  according 
to  its  fineness  ;  or  else  the  rock-fragments  and  cinders 
are  tlirown  up,  and,  falling  again  repeatedly,  may  be 
triturated  into  dust  or  ashes.  The  finest  rock-dust  hang- 
ing in  the  air  is  called  smoke;  the  same,  fallen  to  the 
earth,  ashes.     Volcanic  ashes,  wet  with  water  and  con- 


IGNEOUS  AGENCIES.  139 

solidated  either  on  the  spot  or  after  transportation  and 
sorting,  is  called  tufa. 

Physical  Conditions  of  Lava. — If  lava  cools  very 
slowly,  the  minerals  of  which  it  is  composed  separate  and 
crystallize  more  or  less  perfectly.  This  is  stony  lava.  If 
it  cools  rapidly,  it  forms  volcanic  glass.  If  the  volcanic 
glass  be  full  of  vapor-bubbles,  it  forms  scoria.  If  volcanic 
ashes  mixed  with  water  solidifies,  it  makes  tufa.  Thus 
there  are  four  physical  states  in  which  we  find  lava,  viz., 
stony,  glassy,  scoriaceous,  and  tufaceous. 

Classification  of  Hardened  Lavas. — Hardened  lava 
consists  essentially  of  two  principal  minerals,  viz.,  feld- 
spar and  augite.*  If  the  former  predominate,  it  is  called 
feldspathic  ;  if  the  latter,  augitic  lava.  These  two  min- 
erals are  often  not  detectable  except  with  the  microscope, 
and  yet  the  two  kinds  of  lavas  may  usually  be  distin- 
guished by  the  eye.  The  lighter  colored  and  lighter 
weighted  are  usually  feldspathic  ;  the  darker  and  heavier, 
augitic.  The  feldspathic  lavas  are  said  to  be  acidic  j  the 
augitic,  basic.  Both  of  these  kinds  take  on  the  four 
physical  states  mentioned  above.  Feldspathic  lava,  in 
the  stony  condition,  is  trachyte;  in  glassy  condition, 
obsidian ;  in  scoriaceous  condition,  pumice ;  in  tufa- 
ceous, the  light-colored  tufas.  Augitic  lava,  if  stony,  is 
basalt  J  if  glassy,  pitchstone ;  if  scoriaceous  and  tufa- 
ceous, blach  scoricB  and  tufas. 

Gases  and  Vapors. — The  gases  ejected  from  vol- 
canoes are  steam,  chlorhydric  acid,  sulphurous  acid, 
sulphhydric  acid,  and  carbonic  acid  (H^O,  HCl,  SO^, 
H,S,  CO,).  The  first  three  are  characteristic  of  true 
eruptions,  the  others  of  feeble,  secondary  volcanic  activ- 
ity. Of  all,  steam  is  by  far  the  most  abundant.  In  vol- 
canoes of  the  explosive  type  the  quantity  of  steam  is 
often  enormous.  This  fact  strongly  suggests  this  vapor 
as  the  main  agent  of  eruption.  Flames  are  often  spoken 
*  The  pupil  ought  to  be  shown  specimens  of  these  minerals. 


140 


DYNAMICAL   GEOLOGY. 


of  in  eruptions.  It  is  possible  that  there  may  be  some- 
times feeble  llame  from  the  combustion  of  11  or  H^S,  but 
probably  the  so-called  llame  is  nothing  else  than  the 
ruddy  reflection  of  the  glowing  liquid  in  the  crater  upon 
the  smoke  and  cloud  hanging  in  the  air. 

Formation  of  Volctinoes  and  their  Structure. — It 
is  now  generally  admitted  that  volcanic  cones  are  built 
up  mainly  by  their  own  eruptions.  On  this  view,  their 
origin  and  mode  of  growth  may  be  briefly  described  as 
follows  :  1.  The  increase  of  heat  (by  causes  which  we  lit- 


PlG.  71.— Section  across  Hawaii.    L,  Manna  Loa  ;  K,  Manna  Kea. 


He  understand)  at  the  focus  of  the  volcano  thins  the  crust 
in  that  pointy  until  it  gives  way,  and  the  melted  matter  is 
outpoured  on  the  surface  around  the  opening.  2.  With 
every  eruption  the  accumulated  material  rises  higher  and 
spreads  farther  ;  and  thus  a  conical  mound  is  formed. 
The  shape  of  this  mound  will  depend  on  the  kind  of  mat- 
ter erupted.     If  it  be  very  liquid  lava,  it  will  spread  far, 

and  the  cone  will  be 
low  in  proportion  to  the 
base,  as  in  the  Hawaiian 
volcanoes  (Fig.  71)  ;  but 
if  the  material  be  cin- 
ders, these  will  pile  up 
into  a  steep  cone  (Fig. 
72).  The  repeated  lay- 
ers of  lava  or  cinders  produce  a  stratified  appearance  ; 
but  this  must  not  be  confounded  with  true  stratification. 
3.  With  every  eruption,  the  eruptive  throes  split  the 
sides  of  the  cone  with  radiating  cracks,  which,  filling 
with  liquid  and  hardening,  form  radiating  rocky  ribs 
called  dikes  (Fig.  73),  and  these  bind  the  lava  or  cinder 


Fiti.  72.— Section  of  cinder  cone. 


IGNEOUS  AGENCIES, 


141 


layers  into  a  stronger  mass.     4.  When  the  cone  grows 
very  high,  eruptions  will  take  place  through  these  fis- 


FiG.  73.— Dikes  in  Etna. 

sures,  as  well  as  from  the  top  crater,  and  thus  will  be 
formed  secondary  cones  or  monticles.  5.  If  any  of  these 
monticles  cease  to  erupt,  they  will  be  covered  up  by  ejec- 
tions from  the  main  crater  or  other  secondary  craters. 
All  these  facts  are  shown  in  Fig.  74.     6.  From  time  to 


Fig.  74.— Ideal  section  of  a  volcano,     ss,  original  surface ;  mm,  monJicles ;  m'm\ 
extinct  monticles  ;  cr,  cr,  original  stratified  crusti 


142 


DYNAMICAL   GEOLOGY. 


time,  at  very  long  intervals,  there  occur  very  great  erup- 
tions. If  the  volcano  be  of  the  quiet  type,  the  whole  top 
of  the  cone  is  melted,  and,  after  eruption,  is  ingulfed ; 
or  if  of  the  explosive  type,  the  whole  top  of  the  cone  is 
blown  into  the  air,  and  the  mountain  is  disemboweled. 
In  either  case  a  yawning  chasm  many  miles  in  extent  is 
left.  7.  Within  this  great  crater,  by  subsequent  erup- 
tions, is  built  up  a  smaller  cone,  and  within  this  again 
often  still  smaller  cones.  Thus  volcanoes  often  have 
about  their  present  eruptive  cone  a  great  surrounding 
rampart.  This  rampart  is  the  remains  of  the  great 
crater.     In  Vesuvius  (Fig.  75),  Mount  Somma,  s,  is  the 


Fig.  75.— Section  of  Vesuvius,    vv,  Vesuvius  cone  ;  s.  Mount  Somma  ;  «',  other  side 
of  Somma  overflowed  by  lava  from  Vesuvius. 

remains  of  such  a  great  crater,  the  other  side  of  it  being 
broken  down,  and  now  covered  by  flows  from  the  present 
crater. 

Crater  Lake. — An  excellent  example  of  this  structure 
is  found  in  Crater  Lake.  This  beautiful  lake,  with  its 
splendid  blue  waters,  occupies  a  yawning  chasm  on  the 
top  of  an  extinct  volcano  in  southern  Oregon.  The  lake 
itself  is  about  six  miles  in  diameter  and  2,000  feet  deep 
(the  deepest  lake  on  the  American  continent),  and  is  sur- 
rounded by  almost  perpendicular  walls,  1,000  to  2,200  feet 
high.  From  the  midst  of  the  blue  waters,  but  nearer  one 
side,  there  rises  a  beautiful  island  (Wizard  Isle),  800 
feet  high,  which  is  in  fact  a  cinder  cone  with  a  small 
crater  atop.  Fig.  76  gives  an  ideal  section  of  the  lake 
and  island,  and  also,  in  dotted  outline,  the  supposed  form 


IGNEOUS  AOENCIES.  143 

and   height  of  the   original   volcano  before  ingulfment. 
This  former  volcano  has  been  named  Mt.  Mazama. 


\ 


Fig.  76.— Ideal  section  of  Crater  Lake,  Mt.  Mazama,  and  Wizard  Isle.    (After  Diller.) 

Age  of  Volcanoes. — From  the  progressive  manner  in 
which  volcanoes  grow,  it  would  seem  that  we  may  esti- 
mate their  age.  Such  estimates,  it  is  true,  must  be  very 
rough,  yet  they  are  useful  in  familiarizing  the  mind  with 
the  idea  of  the  great  amount  of  time  necessary  to  account 
for  geological  phenomena.  For  this  purpose  we  will  use 
Etna.  There  have  been,  indeed,  other  volcanic  eruptions 
great  enough  to  build  this  mountain  at  once,  but  the 
eruptions  of  Etna  itself  have  been  very  regular  and  mod- 
erate. 

Etna  is  11,000  feet  high,  and  about  thirty  miles  in  di- 
ameter at  its  base.  We  will  take  its  circumference  at  one 
hundred  miles.  Now,  a  lava-stream  of  triangular  shape, 
one  foot  thick,  reaching  to  the  base,  and  one  mile  wide, 
would,  we  believe,  be  an  average  eruption.  It  would 
cover  seven  square  miles,  one  foot  deep,  and  would  be 
equal  to  more  tlian  200,000,000  cubic  feet.  It  would 
take  one  hundred  such  eruptions  to  raise  the  whole 
mountain-surface  one  foot.  Taking  one  such  eruption 
every  year  (eruptions  of  Etna  for  the  last  2,000  years  have 
been  but  one  in  twenty-five  years),  it  would  take  a  century 
to  raise  the  mountain-surface  one  foot.  But  there  is  a 
gorge  cut  into  the  side  of  this  mountain,  revealing  3,000 
feet  of  lava-layers.  To  have  built  up  these  3,000  feet 
would  require  300,000  years.     That  we  have  been  moder- 


144 


DYNAMICAL   GEOLOGY, 


ate  in  our  estimate  is  shown  bj  the  fact  that  there  are 
linown  on  the  flanks  of  Etna  hiva-flows  2,000  years  old, 
which  are  still  not  covered  by  subsequent  flows.  We  are 
justified,  then,  in  saying  that  Etna  is  probably  much 
more  than  300,000  years  old.  But  the  birth  of  Etna  is  a 
very  recent  geological  event,  for  it  stands,  and  has  been 
built  up,  on  the  latest  tertiary  formation. 


Cause  of  Volcanic  Eruptions. 

This  question  is  still  very  obscure.  There  are  two 
things  to  be  explained,  viz.,  volcanic  force  and  volcanic 
heat — the  force  necessary  to  raise  lava  to  the  lip  of  the 
crater,  and  the  heat  necessary  to  melt  the  lava. 

{a.)  Force. — If  we  consider  the  height  of  volcanic 
cones,  we  shall  be  better  able  to  appreciate  the  greatness 
of  the  force.  In  the  accompanying  table  we  give  the 
heights  of  some  well-known  volcanoes  and  the  pressure  in 
atmospheres  (one  atmosphere  =  fifteen  pounds  per  square 
inch,  or  one  ton  per  square  foot)  necessary  to  raise  lava 
(taking  the  specific,  gravity  at  2.8)  to  the  lip  of  the  crater. 
It  is  true  lava  is  sometimes  foamy,  and  therefore  lighter, 
but,  on  the  other  hand,  we  have  taken  the  focus  of  vol- 
canoes at  sea-level,  while  it  is  probably  much  deeper,  and 
have  supposed  the  force  only  sufficient  to  raise  to  the  lip 
of  the  crater,  whereas  it  often  ejects  with  violence  many 
thousand  feet  in  the  air. 


NAME. 

Height. 

Pressure  in 
atmospheres. 

Vesuvius 

3,900  feet 
11,000     " 
13,800     " 
19,660     " 

325 

Etna 

920 

Mauna  Loa .  . . 

Cotopaxi 

1,150 
1,688 

What,  then,   is  the  agent  of  this  great  force  ?     It  is 
believed  that  it  is  the  elastic  force  of  compressed  gases 


IGNEOUS  AGENCIES.  145 

and  vapors,  especially  steam.  The  power  of  these  agents 
is  w  ill  known  ;  and  gas  and  steam  issne  in  immense  quan- 
tities during  eruptions,  especially  of  the  explosive  type. 
On  this  point  there  is  little  difference  of  opinion. 

{jb.)  Heat. — But  the  cause  of  the  heat  necessary  to  fuse 
the  rocks  is  one  of  the  most  difficult  of  all  questions  con- 
nected with  the  physics  of  the  earth.  By  most  geologists 
it  is  thought  to  be  connected  with  the  primal  heat  of  the 
earth,  and  the  supposed  universal  melted  condition  of 
the  interior.  This  view  assumes  («)  that  the  earth  was 
once  an  incandescent,  fused  mass.  This  is  almost  cer- 
tainly true  ;  {h)  that  in  cooling  it  formed  a  crust,  which 
thickened  by  additions  to  its  inner  face,  until  it  is  now 
about  thirty  miles  thick  ;  (c)  and  that  this  limit  between 
the  solid  crust  and  melted  interior  is  the  place  of  the 
focus  of  volcanoes.  There  are  many  difficulties  in  the 
way  of  acceptance  of  this  view,  some  of  which  are  given 
on  page  132. 

All  other  theories  regard  the  melted  matter  as  local, 
but,  as  to  the  cause  of  the  fusion,  there  is  yet  great  diver- 
sity of  view.  Some  attribute  it  to  chemical  action  ;  some 
to  mechanical  crushing.  It  must  be  remembered  in  this 
connection,  however,  that  in  some  cases,  at.  least,  the 
amount  of  heat  required  is  not  more  than  800""  F.,  for  in 
some  lavas  the  fusion  is  hy dr other mal,  and  in  all  cases 
the  access  of  water  seems  necessary  to  supply  the  force. 

Secondary  Volcanic  Phenomena. 

There  are  many  phenomena  which  linger  after  the  true 
eruptions  have  ceased.  The  chief  of  these  are  hot  springs, 
carbonated  springs,  lime-depositmg  springs,  solfataras, 
fumaroles,  mud-volcanoes,  and  geysers.  These  all  seem  to 
be  the  result  of  circulation  of  water  through  lavas 'which 
still  retain  their  heat,  and  are  therefore  properly  called 
secondary  volcanic  phenomena.  The  lavas,  outpoured  by 
primary  or  true  eruptions,  remain  hot  in  their  interior  for 

Lb  Contk.  Geol.  10 


146  '  DYNAMICAL   OEOLOOY. 

an  indefinite  time.  If  waters,  percolating  through  these, 
come  up  again  after  taking  up  only  heat,  they  form  hot 
springs.  If,  in  addition,  they  take  up  CO.^,  they  form 
carbonated  springs.  If  lime  be  taken  and  deposited  on 
the  surface,  they  form  lime-depositing  springs  (p.  73). 
If  the  heat  be  great,  so  that  vapors  are  given  off  and  con- 
densed as  clouds,  they  are  called  f  umaroles.  If  the  waters 
contain  H^S,  and  AlkS,  they  are  called  solfataras.  If 
mud  is  brought  up  and  deposited  about  the  vent,  they 
are  mud-volcanoes.  Finally,  if  the  springs  are  periodi- 
cally and  violently  eruptive,  they  are  called  geysers.  The. 
only  variety  of  these  springs  which  need  detain  us  here  ia 

Geysers. 

Geysers  may  be  defined  as  periodically  eruptive  springs 
They  seem  also  usually,  if  not  always,  to  deposit  silica. 
They  are  found  only  in  Iceland,  in  New  Zealand,  and 
in  Yellowstone  Park.  The  so-called  California  geysers 
are  solfataric  fumaroles.  Steamboat  Springs  in  Nevada 
may  possibly  be  classed  with  geysers,  but  their  erup- 
tions are  feeble.  The  phenomena  of  true  geysers  are  so 
splendid  that  a  somewhat  full  account  of  them  is  neces- 
sary. As  they  were  first  studied  in  Iceland,  and  the  cause 
'  of  their  eruption  was  first  understood  there,  we  will  speak 
of  these  first. 

Geysers  of  Iceland. — Iceland  may  be  briefly  described 
as  a  plateau  2,000  feet  high,  studded  with  volcanic  peaks, 
with  margin  sloping  gently  to  the  sea.  Only  the  marginal 
area  is  to  any  extent  inhabited.  The  interior  is  a  scene 
of  desolation,  where  every  form  of  volcanic  phenomena 
exists  in  the  greatest  activity — volcanoes,  hot  springs, 
boiling  springs,  fumaroles,  solfataras,  and  geysers.  Of 
these  last  there  are  very  many  in  various  degrees  of 
activity.  The  most  celebrated  of  these  is  the  Greaf 
Oeyser, 


IGNEOUS  AGENCIES.  147 

Great  Geyser. — This  is  a  low  mound,  with  a  basin- 
shaped  depression  at  top,  from  the  bottom  of  which  de- 
scends a  tube  or  well  to  unknown  depth,  but  may  be 
sounded  to  eighty  feet  or  more.  The  basin  is  fifty  feet 
across,  and  the  tube  or  throat  ten  feet  in  diameter  at  the 
top  but  narrowing  downward.  Both  the  basin  and  the 
throat  are  lined  with  silica  deposited  from  the  water,  and 
doubtless  the  mound  itself  was  built  up  by  similar  deposits. 
In  the  intervals  between  eruptions  the  basin  is  filled  to 
near  the  brim  with  water  at  180°. 

Phenomena  of  an  Eruption. — As  the  time  for  the 
eruption  approaches,  the  first  thing  observed  is  a  series  of 
explosions  in  the  bottom  of  the  throat  like  subterranean 
cannonading  ;  then  bubbles  of  vapor  are  seen  to  rise  and 
burst  on  the  surface  ;  then  the  water  of  the  surface  bulges 
up  and  overflows.  Immediately  thereafter  the  whole  of 
the  water  in  the  throat  and  basin  is  ejected  with  violence 
one  hundred  feet  into  the  air,  forming  a  fountain  of  daz- 
zling splendor,  followed  by  the  roaring  escape  of  steam. 
As  the  water  falls  back,  it  is  again  ejected,  and  the  foun- 
tain continues  to  play  several  minutes  until  the  steam  has 
all  escaped  and  the  water  partly  cooled ;  then  all  is  quiet 
again  until  another  eruption.  The  interval  between  erup- 
tions is  irregular.  An  eruption  may  be  brought  on  pre- 
maturely by  throwing  large  stones  down  the  throat  of  the 
geyser. 

Yellowstone  Geysers. — But  in  splendor  of  eruption! 
the  Icelandic  geysers  are  far  surpassed  by  those  of  Yellow- 
stone Park.  This,  like  Iceland,  is  a  volcanic  region,  but, 
unlike  Iceland,  primary  volcanic  pheaomena  are  all  ex- 
tinct. The  geyser  phenomena  here  occur  in  a  narrow 
valley  surrounded  on  all  sides  with  volcanic  rocks  of  great 
thickness,  of  comparatively  recent  origin,  and  doubtless, 
therefore,  still  hot  in  their  interior.  In  this  little  valley 
there  are  no  less  than  10,000  vents  of  all  kinds,  hot  springs, 
boiling  springs,  mud-volcanoes,  lime-depositing  springs. 


148 


D  YNAMICAL    GEOLOG  Y. 


and  geysers.     On  Gardiner^s  River  the  vents  are  mostly 
hot   carbonated   springs,    depositing   lime ;    on   Firehole 
Eiver  they  are  geysers,  depositing  silica.     In  Yellowstone 
Park  alone  there  are  in 
all  3,000  vents,  of  which 
sixty-two     are     eruptive 
geysers.       In    the    lime 
carbonate  springs  the  de- 
posits  on  hillsides  have 
given  rise  to  a  succession 
of  terraces  (Fig.  41,  page 
75),   and  sometimes  the 
water    descending 
through  a  succes- 
sion of  pools  from 
terrace  to  terrace 


Fig.  77.— Deposits  from  carbonated  springs. 

gives  rise  to  beautiful  stalactitic  forms  (Fig.  77).  In 
the  geysers  the  hot  alkaline  waters  collect  in  pools, 
and  deposit  the  silica  first  in  a  gelatinous  condition, 
which  afterward  concretes  into  all  kinds  of  fantastic  forms 
(Fig.  78).     The  deposit  immediately  about  the  eruptive 


IGNEOUS  AGENCIES. 


149 


150 


nVNAMICAL   GEOLOGY. 


vents   builds   up  moundlike,  hivelike,  and  chimneylike 
forms   (Fig.   79).     The   silica-charged  waters,  trickling 


Fig.  79.— Crater  of  Castle  Geyser,  Yellowstoue  Park. 

slowly  over  the  mounds,  give  rise  by  deposit  to  patterns 
of  exquisite  and  delicate  beauty,  compared  by  Hayden  to 
embroidered  lace-work  with  edging-fringe  and  pendent 
tassels,  and  studded  with  pearls.  Similar  deposits  are 
formed  also  in  New  Zealand  ;  we  give  an  example  in  Fig. 
80.  Only  a  few  of  the  grandest  of  these  geysers  can  be 
mentioned  : 

1.  The  Grand  Geyser  throws  up  a  column  of  water  six 
feet  in  diameter  to  the  height  of  200  feet,  while  the  steam 
ascends  1,000  feet  or  more.  The  eruption  is  repeated 
every  thirty-two  hours,  and  lasts  twenty  minutes. 

2.  The  Giant  (Fig.  81)  throws  a  column  five  feet  in 
diameter  140  feet  in  the  air,  and  plays  continuously  for 
three  hours. 

3.  The  Giantess,  the  greatest  of  all,  throws  up  a  huge 


IGNEOUS  AGENCIES. 


151 


column  twenty  feet  in  diameter  to  the  height  of  sixty  feet, 
and  through  this  great  mass  it  shoots  up  several  lesser  jets 


Fig.  80.— Pink  terraces,  New  Zealand.     (After  Peale.) 


to  the  height  of  250  feet.     It  erupts  about  once  in  eleven 
hours,  and  plays  twenty  minutes. 

4.  The  Beehive,  so  called  from  the  shape  of  its  mound, 
shoots  up  a  splendid  column  two  to  three  feet  in  diameter 
to  the  height  of  219  feet,  and  plays  fifteen  minutes  (Fig. 
82). 

5.  Old  Faithful,  so  called  from  the  frequency  and  regu- 
larity of  its  eruptions,  throws  up  a  column  six  feet  in 
diameter  to  the  height  of  100  to  150  feet,  and  plays  fifteen 
minutes  (Fig.  83). 

Cause  of  Geyser  Eruption."- — This  maybe  explained, 
in  a  very  general  way,  as  follows  :  Experiments  show  that 
the  lieat  of  the  water  rapidly  increases  as  we  pass  down  the 
geyser-throat.  There  is  no  doubt,  therefore,  that  in  spite 
of  the  increasing  pressure  (which  raises  the  boiling-point) 

"  For  a  complete  discussion  of  this  interesting  subject,  see  author's 
•'  Elements,"  pp.  99-104. 


152 


DYNAMICAL   GEOLOGY, 


^mm-^' 


111 


ii 


mm:' 


liiilliM 


Fig.  81.— Giant  geyser.    (After  Hayden.) 


IGJSEOUS  AGENCIES. 


153 


\ 


Fig.  82.— Beehive  geyser.     (From  a  drawing  by  Holaies.) 


154 


DYNAMICAL   GEOLOGY. 


the  boiling-point  is  reached  and  a  large  quantity  of  steam 
is  formed  first,  at  some  point  deep  below.  The  water 
above  is  immediately  ejected,  and  the  fountain  continues 


Pig.  83.— Old  Faithful  geyser  in  action.    (After  Hayden.) 

to  play  until  all  the  steam  escapes  and  the  water  is  some- 
what cooled.  Then  all  is  quiet  until  the  water  again 
heats  up  to  the  boiling-point. 


Section  II. — Earthquakes. 

When  we  consider  the  suddenness  with,  which  earth- 
quakes occur,  the  terror  they  inspire,  and  the  place  of 
their  origin,  deep  in  the  interior  of  the  earth,  and  liidden 
from  observation,  it  is  not  surprising  that  we  know  so 


IGNEOUS  AGENCIES.  155 

little  about  their  cause.  In  fact,  until  about  forty  years 
ago  no  attempt  had  been  made  to  study  them  scientifi- 
cally. Now,  however,  it  is  believed  the  foundations  of  a 
true  science  of  earthquakes  (seismology)  have  been  laid, 
and  a  true  progress  has  been  made.  The  basis  has  been 
laid  by  Mr.  Mallet,  and  progress  has  been  made  possible 
by  the  use  of  self -registering  seismometers. 

Frequency  of  Earthquakes. — The  slow  development 
of  earthquake-science  is  not  due  to  want  of  material,  but, 
as  has  already  been  stated,  partly  to  the  difficulty  of  the 
subject,  and  partly  to  the  terror  produced — unfitting  the 
mind  for  scientific  observation.  The  earthquake  catalogue 
of  Alexis  Perrey  records  18,000  in  thirty  years  (1843- 
1873)^  or  nearly  two  a  day.  When  we  remember  that 
three-fourths  of  the  earth's  surface  is  covered  with  the 
sea,  that  a  large  portion  of  the  land-surface  is  inhabited 
by  uncivilized  races,  and  that  even  in  civilized  countries 
many  slight  tremors  are  unrecorded,  it  will  not  seem 
extravagant  to  say  that,  probably,  there  is  not  an  hour  of 
any  day  in  which  the  earth  is  not  shaking  in  some  portion 
of  its  surface. 

Phenomeua  of  an  Earthquake. — In  brief,  the  phe- 
nomena of  an  earthquake  are  :  1.  Sounds,  sometimes  like 
underground  cannonadi7ig  ;  sometimes  a  hollow  rumbling, 
or  clashing,  or  griyiding.  2.  Accompanying,  or  immedi- 
ately succeeding,  comes  the  movement  of  the  earth,  as  a 
slight  tremor,  or  as  a  violent  shaking  ;  in  extreme  cases, 
so  violent  that  the  houses  of  whole  cities  are  shaken  down, 
like  card-houses  of  children,  and  bodies  on  the  surface  are 
thrown  up  a  hundred  feet  into  the  air,  as  at  Riobamba  in 
1797.  3.  As  to  direction,  the  rfiovement  may  be  up  and 
down,  or  from  side  to  side,  or  partaking  of  both,  i.  e., 
obliquely,  or  it  may  be  rotating  or  twisting,  as,  for  ex- 
ample, when  chimney-tops  are  twisted  about  without 
being  upset,  or  wardrobes  and  bureaus  turned  about  before 
upsetting.      4.  One  thing  is  always  observed  and  is  of 


loG  DYMAMICAL   GEOLOGY. 

primary  importance,  viz.,  that  the  shake  does  not  occur 
everywhere  at  the  same  time,  but  on  the  contrary  appears 
first  at  one  place  and  spreads  thence  in  all  directions, 
precisely  like  a  system  of  waves  when  a  stone  is  thrown 
into  the  water.  This  point  of  first  appearance  is  called 
the  ^'  epicentrum,"  because  it  is  immediately  above  the 
origin.  The  violence  of  the  earthquake  is  greatest  there, 
and  thence  decreases  precisely  like  a  system  of  widening 
circular  waves. 

Velocity  of  Shock  and  of  Transit. — The  velocity  of 
the  spread  from  the  center  or  velocity  of  travel  (transit) 
must  be  carefully  distinguished  from  the  velocity  of  the 
earth-movement  (shock).  There  is  no  close  relation  be- 
tween these.  We  may  best  illustrate  this  by  water-waves. 
Suppose  we  are  in  a  boat  on  the  surface  of  a  bay  traversed 
by  long,  low  swells.  As  each  sw^ell  passes  under  us,  we 
are  slowly  heaved  up  and  slowly  let  down  again,  but  the 
waves  are  here,  there,  and  away  with  great  velocity.  The 
velocity  of  oscillation  is  small,  the  velocity  of  transit 
is  great.  But  if  the  surface  of  the  bay  be  agitated  by 
short,  high  waves,  the  oscillation  or  shaking  is  more  rapid, 
but  the  transit  is  comparatively  slow.  So  in  earthquakes, 
the  movement  may  be  only  a  slow  heaving  up  and  down, 
or  swinging  back  and  forth,  and  yet  this  movement  may 
travel  from  place  to  place  with  great  velocity.  Now,  as 
in  water-waves  generated  by  a  stone  thrown  in  still  water, 
so  in  earthquakes,  the  velocity  and  amount  of  movement 
(which  is  equivalent  to  the  wave-height)  is  greatest  at  the 
center  (epicentrum),  and  diminishes  as  it  spreads,  but  the 
velocity  of  the  transit  or  travel  is  nearly  or  quite  uniform. 

Now,  the  velocity  of  transit  has  been  determined  in 
many  earthquakes  by  noting  the  time  of  arrival  at  different 
places.  It  varies  with  the  kind  of  rock,  being  greatest  in 
the  hardest,  and  also  with  the  depth  of  the  origin,  being 
greater  for  very  deep  earthquakes.  In  some  cases  it  is 
only  ten  miles  per  minute  ;  sometimes  fifteen,  twenty. 


IGNEOUS  AGENCIES.  157 

thirty  miles  per  minute,  or  even  much  more.-  Sometimes 
the  spread  is  equally  rapid  in  all  directions,  and  the  spread- 
ing wave  is  circular,  or  nearly  so  ;  sometimes  it  is  more 
rapid  in  one  direction  than  another,  and  the  spreading 
wave  is  elliptical. 

Cause  of  Earthquakes. — The  origin  of  earthquakes 
being  deep  beneath  the  surface  and  hidden  from  obser- 
vation, their  cause  is  very  obscure.  Yet  their  association 
with  other  forms  of  igneous  agency  suggests  prolaUe 
causes  : 

1.  Volcanic  eruptions,  especially  of  the  explosive  type, 
are  always  accompanied  by  slight  and  sometimes  by  seri- 
ous earthquakes.  This  fact  suggests  the  sudden  formation 
of  gases  or  the  sudden  collapse  of  vapors  as  a  possible 
cause.  On  this  view  an  earthquake  would  be  like  the 
earth-jar  produced  by  a  mine-explosion,  or  by  the  explo- 
sion of  large  quantities  of  gunpowder  or  nitro-glycerine. 

2.  But  great  earthquakes  are  of tener  associated  with 
bodily  movements  of  extensive  areas  of  the  earth-crust. 
Thus,  for  example,  in  1835,  after  a  severe  earthquake  on 
the  western  coast  of  South  America,  it  was  found  that  the 
whole  coast-line  of  Chili  and  Patagonia  was  raised  from 
two  to  ten  feet  above  sea-level.  Again,  in  1822,  the  same 
phenomenon  was  observed  in  the  same  region  after  a  great 
earthquake.  Again,  in  1810,  after  a  severe  earthquake 
which  shook  the  delta  of  the  Indus,  a  tract  of  land  fifty 
miles  long  and  sixteen  miles  wide  was  raised  ten  feet,  and 
an  adjacent  area  of  2,000  square  miles  was  sunk,  and 
became  a  lagoon.  In  commemoration  of  the  wonderful 
event,  the  elevated  tract  was  called  Ullah  bund,  or,  the 
mound  of  God.  Again,  in  1811,  a  severe  earthquake — 
perhaps  the  severest  (except  the  Charleston  earthquake  of 
August,  1886)  ever  felt  in  the  United  States— shook  the 
valley  of  the  Mississippi.  Coincidently  with  the  shock, 
large  areas  of  the  river-swamp  sank  bodily,  and  have  ever 
since  been  covered  with  water.    In  commemoration  of  the 


158  DYNAMICAL   GEOLOGY. 

event,  this  area  is  still  called  the  simhen  country.  In  all 
these  cases,  probably,  and  in  the  last  two  certainly,  there 
was  a  great  fissure  of  the  earth-crust,  and  a  slipping  of 
one  side  on  the  other. 

Now,  these  facts  suggest  another  and,  we  believe,  a 
more  probable  cause  of  earthquakes.  It  is  Avell  known 
that  there  are  operating  within  the  earth  forces  elevating 
or  depressing  or  crushing  together  portions  of  the  crust. 
y^e  will  discuss  the  nature  of  these  forces  in  Part  II. 
Suffice  it  to  say  now  that  it  is  in  this  way  that  continents 
are  elevated  and  mountain-ranges  are  formed.  Now, 
suppose  such  forces  operating  to  raise  or  depress  large 
areas  of  the  crust — e.  g.,  the  southern  end  of  South 
America — it  is  evident  that,  the  interior  forces  lifting  and 
the  stiif  crust  resisting,  there  would  come  a  time  when 
the  crust  would  break — i.  e.,  form  a  great  fissure.  Such 
a  sudden  break  would  produce  an  earth-jar  which  would 
propagate  itself  from  the  fissure  as  focus  in  all  directions 
as  an  earthquake.  Or,  again,  after  such  a  fissure  is  formed, 
the  two  walls  may  at  any  time  slip  on  each  other  and  pro- 
duce an  earth- jar.  Now,  this  is  not  mere  speculation. 
We  find  such  great  fissures  intersecting  the  earth  in  many 
places  ;  they  break  through  miles  of  thickness  of  rock, 
and  in  many  cases  the  two  walls  are  slipped  on  each  other 
several  thousand  feet  vertically.  It  is  almost  certain  that 
earthquakes  are  produced  hy  the  formation  or  the  slipping 
of  such  fissures.  In  1873  there  was  a  severe  earthquake 
in  Inyo  County,  California,  just  at  the  eastern  base  of  the 
Sierra.  Now,  there  is  on  that  side  of  the  range  a  great 
fissure  and  a  slip  of  several  thousand  feet.  It  is  almost 
certain  that  the  earthquake  was  produced  by  a  slight 
readjustment  of  the  position  of  the  walls  of  this  fissure. 
Moreover,  the  thorough  investigations  very  recently  of 
several  earthquakes  have  seemed  to  establish  the  fact  that 
they  originated  in  the  formation  or  the  readjustment  of  a 
fissure. 


lONEOrS  AGENCIES. 


159 


Nature  of  Earthquake- Waves. — In  any  case,  it   is 

evident  that  an  earthquake  is  produced  by  concussion  of 
some  kind  somewhere  in  the  interior  of  the  earth,  usually 
at  a  depth  of  from  six  to  ten  miles.  The  concussion 
gives  rise  to  a  series  of  elastic  earth-waves,  spreading  in 
all  directions  spherically,  like  sound-waves,  until  they 
reach  the  surface,  and  then  spread  in  all  directions  on  the 
surface  as  a  circular  wave,  as  in  Fig.  84.     The  interior 


40  m' 


Fio.  84.— Section  and  perspective  of  a  portion  of  the  earth's  crii8t  shaken  by  an 
earthquake,  showing  origin,  x;  section  of  the  spherical  waves,  a',  b',  c\  etc.,  and 
perspective  of  the  outcropping  surface  waves,  a,  b,  c,  etc. 

point  of  origin  {x)  is  called  the  focus,  or  centrum ;  the 
point  of  first  emergence  {a),  the  epicentrum.  It  is  the 
passage  of  a  series  of  these  circular  waves  beneath  the 
feet  of  the  observer  at  any  point  {d)  that  gives  rise  to 
the  actual  observed  phenomena ;  so  that  the  scientific 
discussion  of  earthquake  phenomena  is  little  else  than 
the  discussion  of  such  earth-waves  emerging  and  spreading 
on  the  surface. 

Earthquakes  occurring-  beneath  the  Sea. — We  have 
thus  far  spoken  of  earthquakes  occurring  beneath  the 
land  ;  but  three  fourths  of  the  earth-crust  is  covered  with 
water,  and  therefore  it  is  probable  that  the  larger  number 
of  earthquakes  have  their  origin  beneath  the  sea-bed. 
Besides,  as  we  shall  see  hereafter  in  treating  of  mountain- 
chains,  marginal  sea-bottoms  are  particularly  liable  to 
movements.  When  an  earthquake  occurs  beneath  the 
sea-bed,  there  are  some  additional  phenomena,  which 
must  now  be  discussed. 


IGO  DY^'AMICAL  GEOLOGY. 

Suppose,  then,  a  concussion,  from  any  cause,  beneath 
the  sea-bed.  There  would  be  formed,  as  before,  about 
the  focus,  a  series  of  spherical  earth-waves,  which,  by  en- 
largement, would  emerge  on  the  surface  of  the  sea-bed 
as  circular  surface-waves.  These,  spreading  beneath  the 
sea,  would  reach  the  nearest  shore,  and  produce  their  de- 
structive eifects  there.  Some  time  afterward,  perhaps 
a  half -hour  or  more,  there  comes  rolling  in  on  shore  a 
prodigious  water  wave,  or  perhaps  a  series  of  water  waves, 
thirty  to  sixty  feet  high,  deluging  the  whole  shore  region, 
and  completing  the  destruction  commenced  by  the  earth- 
wave. 

The  Great  Sea  Wave. — This  very  destructive  accom- 
paniment of  earthquakes  occurring  beneath  oifshore  sea- 
beds  may  be  explained  as  follows  :  The  bed  of  the  sea 
at  the  epicentrum  is  lifted  up  perhaps  several  times. 
This  lifts  the  whole  sea  water  above,  so  that  the  surface 
is  raised  into  a  water  mound.  This  mound  immediately 
sinks  as  much  below  the  sea  level  as  it  was  before  raised 
above  it,  and  thus  gives  origin  to  a  circular  water  wave 
(or  series  of  such  waves)  which  spreads  exactly  like  any 
other  water  wave,  growing  lower  as  its  spreads,  until  it 
breaks  on  the  nearest  shore.  Out  at  sea  such  great  low 
waves  would  pass  under  a  ship  unobserved,  heaving  it 
slowly  up  and  letting  it  down  again.  But  when  they 
approach  shore,  on  account  of  their  great  size,  often  fifty 
feet  high  and  one  hundred  to  two  hundred  miles  across 
the  base,  they  rush  forward  as  a  tide  fifty  feet  high  and 
devastate  the  whole  coast  within  their  reach.  They  are, 
therefore,  sometimes  called  tidal  waves,  although  they 
have  nothing  to  do  with  tides.  Though  originating  at  the 
same  place,  the  great  sea  wave  moves  much  less  rapidly 
than  the  earth-wave,  and  therefore  reaches  the  shore  later. 

Examples  of  the  Great  Sea  Wave. — 1.  In  1755  a 
terrible  earthquake  destroyed  Lisbon,  and,  it  is  said,  forty 
thousand   people.      The   focus   of   this   earthquake   was 


JONEOUS  AGENCIES.  161 

beneath  the  sea-bed,  perhaps  one  hundred  miles  off  shore. 
The  arrival  of  the  earth-wave  shook  down  the  houses. 
Then,  after  a  half -hour,  when  all  was  quiet,  there  came 
great  sea  waves  sixty  feet  high  and  completed  the  destruc- 
tion of  the  city.  These  waves  were  sixty  feet  high  at 
Lisbon,  thirty  feet  at  Cadiz,  eighteen  feet  at  Madeira,  and 
five  feet  on  the  coast  of  Ireland.  They  were  also  felt  on 
the  coast  of  Norway  and  on  the  West  India  Islands,  after 
having  traversed  the  breadth  of  the  Atlantic. 

2.  In  1854  an  earthquake  shook  the  coast  of  Japan. 
A  half -hour  afterward  a  great  wave,  thirty  feet  high,  came 
in  and  swept  the  town  of  Simoda  clean  away.  The  epi- 
centrum  was  probably  a  hundred  miles  off  shore.  The 
wave,  spreading  in  all  directions,  was  highest  on  the  coast 
of  Japan,  because  this  was  near  the  epicentrum.  But  in 
the  other  direction  it  was  observed  at  the  Bonin  Islands 
fifteen  feet  high,  and — after  traversing  the  Pacific  and 
being  nearly  exhausted — on  the  California  coast,  only 
eight  inches  high  at  San  Francisco  and  six  inches  at  San 
Diego. 

3.  In  August,  1868,  a  very  destructive  earthquake 
shook  the  coast  of  Peru,  severest  about  Arica.  The  epi- 
centrum was  not  far  off  shore,  for  in  five  minutes  after- 
ward there  came  in  great  sea  waves  sixty  feet  high  and 
desolated  the  whole  coast,  carrying  ships  far  inland  and 
stranding  them  high  up  on  the  mountain  slopes.  These 
great  waves  were  traced  southward  to  Coquimbo  and  be- 
yond, northward  to  San  Francisco,  Astoria,  and  Sitka, 
southwestward  to  Australia  and  New  Zealand,  and  west- 
ward to  Hawaii  and  Japan,  thus  having  traversed  the 
whole  breadth  of  the  Pacific.  Were  it  not  for  the  obstruct- 
ing continents,  there  is  no  doubt  that  they  would  have 
encompassed  the  earth  in  their  widening  circles. 

In  regard  to  these  waves,  there  are  several  points  worthy 
of  notice  : 

a.  Their  velocity,  though  less  than  that  of  earth-waves, 

Lk  Conte,  Geol.  11 


162  DYNAMICAL   OEOLOQY. 

is  enormously  great  for  water  waves.  The  wave  of  1854 
traversed  the  Pacific,  from  Japan  to  San  Francisco,  a  dis- 
tance of  4,500  miles,  in  about  twelve  hours,  or  at  a  rate 
of -370  miles  an  hour.  The  wave  of  1868  ran  across  the 
Pacific  with  even  greater  speed.  The  reason  of  their 
great  velocity  is  their  enormous  size. 

h.  The  size  of  the  great  sea  wave  is  determined  by  the 
principle  that  every  wave  runs  its  own  length  in  the  time 
of  one  oscillatio7i.  If  a  boat  be  lying  on  smooth  water, 
and  a  series  of  water  waves  passes  under  it,  the  boat  will 
be  moved  up  and  down  once  while  the  waves  run  the 
length  of  one  wave  ;  i.  e.,  from  trough  to  trough.  Now, 
the  time  of  oscillation  of  the  great  sea  waves  of  1854  was 
about  thirty-three  minutes.  If,  then,  the  waves  run  370 
miles  in  an  hour  (60  minutes),  how  much  did  they  run  in 
33  minutes— 60  :  33  :  :  370  :  203.  Therefore,  these  waves 
were  203  miles  from  trough  to  trough. 

c.  The  mean  depth  of  the  ocean  may  be  determined  by 
these  waves.  The  principle  on  which  this  is  done  is  as 
follows  :  Every  one  has  observed  that  waves  coming  in 
from  deep  water  on  to  a  flat,  shelving  shore,  at  a  certain 
depth  begin  to  drag  bottom,  and  ^re  impeded  thereby  ; 
also,  that  the  larger  the  wave,  the  deeper  the  water  in 
which  it  begins  to  drag.  Now,  in  the  case  of  these  enor- 
mous earthquake  sea  waves,  the  ocean  itself  is  not  deep 
enough  to  prevent  them  from  dragging  bottom.  As  they 
run  over  the  sea  their  velocity  is  impeded  everywhere, 
but  more  or  less  according  to  the  varying  depth  of  the 
ocean.  Now,  the  normal  or  unimpeded  velocity  of  a  wave 
may  be  accurately  calculated,  since  it  varies  as  the  square 
root  of  the  wave-length  {v  <x  VL).  Therefore,  the 
amount  of  retardation  will  give  the  depth  of  the  ocean 
over  which  it  passes.  The  mean  depth  of  the  ocean 
between  Japan  and  San  Francisco,  as  thus  determined, 
is  12,000  feet  ;  between  Arica  and  Hawaii  it  is  18,000 
feet. 


IGNEOUS  AGENCIES. 


163 


Determination  of  the  Epicentruni  and  Centrum. 

— By  means  of  seismometers  the  direction  of  the  earth's 
motion  may  be  determined.  If  this  be  taken  in  many 
places^  and  the  lines  of  direction  be  protracted,  they  will 
be  found  to  meet  at  some  point  from  Avhich  all  seem  to 
radiate.  This  is  the  center  of  the  circular  surface-waves 
or  epicentrum.  Or,  by  accurate  clocks  in  many  stations, 
the  tirne  of  arrival  of  the  shock  may  be  recorded.  If, 
now,  we  draw  a  line  through  all  the  places  where  the  time 
of  arrival  was  the  same,  we 
shall  have  a  curve  which 
represents  the  form  of  the 
wave  and  the  center  of 
which,  a,  is  the  epicen- 
trum. Such  lines  of  simul- 
taneous arrival  of  shock 
are  called  coseismal  lines 
{c  s,  Fig.  85). 

The  position  of  the  cen- 
trum or  origin  is  much 
more  difficult  to  find,  but 
has    been    approximately 

found  for  several  earthquakes.  The  general  conclusion 
thus  arrived  at  is  that  an  earthquake  focus  (centrum)  is 
usually  only  six  to  ten  miles  in  depth,  and  that  the  shock 
is  a  jar  produced  by  the  formation  of  a  great  fissure. 

Connection  of  Earthquakes  with  Phases  of  the 
Moon. — By  careful  comparison  of  the  times  of  occur- 
rence of  thousands  of  earthquakes,  it  has  been  shown — 1. 
That  they  are  a  little  more  frequent  when  the  moon 
is  on  the  meridiau  than  when  on  the  horizon.  2.  Also  at 
new  and  full  moon  than  at  half  moons.  3.  Also  when 
the  moon  is  nearest  the  earth  than  when  she  is  farthest 
away.  Now,  these  are  the  times  of  flood-tide,  and  of 
high  flood-tides,  and  of  highest  flood-tides.  Some  have 
imagined   that   these   facts   prove   the   existence  in  the 


Fig.  85. 


164  DYNA3IICAL   GEOLOGY. 

interior  of  the  eiirth  of  a  general  liquid  subject  to  tides. 
But  the  argument  is  evidently  valueless,  for  any  force 
tending  to  lift  and  break  up  the  crust  of  the  earth  would 
be  assisted  by  the  gravitation  or  lifting  power  of  the 
moon  in  passing  the  meridian,  and  this  lifting  power 
would  be  greatest  at  the  times  indicated  above.  Suppose, 
then,  an  interior  force,  tending  to  elevate  and  break  the 
crust,  constantly  increasing  but  resisted  by  the  rigidity  of 
the  crust  :  it  is  evident  that,  when  the  two  forces  are 
nearly  balanced,  the  lifting  force  of  the  passing  moon 
might  well  determine  the  moment  of  fracture.  The 
moon  does  not  produce  the  earthquake,  but  only  deter- 
mines the  moment  of  its  occurrence — only  adds  the  last 
feather  that  breaks  the  camel's  back. 

Connection  with  Season  and  Weather. — By  the 
discussion  of  the  times  of  occurrence  of  a  large  number  of 
earthquakes  it  is  found  that  they  are  a  little  more  fre- 
quent in  winter  than  in  summer.  No  cause  for  this  is 
known. 

Again  :  It  is  a  popular  belief  that  the  occurrence  is 
usually  associated  with  an  oppressive  feeling  of  the  atmos- 
phere, or  with  storms.  These  meteorological  phenomena 
are  usually  attended  with  a  low  condition  of  the  barome- 
ter. Now,  a  low  barometer  means  diminished  pressure  of 
the  atmosphere,  and  this,  again,  might  determine  the 
moment  of  fracture  of  the  crust.  But  this,  like  the 
attraction  of  the  moon,  must  be  regarded,  not  as  the 
cause  of  the  earthquake  (which  undoubtedly  lies  wholly 
within  the  earth  itself),  but  only  as  sometimes  determin- 
ing the  moment  of  its  occurrence. 

Sectiok  III. — Gradual  Oscillations  of  the  Earth- 
Crust. 

The  movements  included  under  this  head  are  on  a 
grand  scale,  perhaps  affecting  whole  continents,  but  usu- 


IGNEOUS  AGENCIES.  165 

ally  so  slow  as  to  escape  popular  observation.  But, 
though  so  inconspicuous,  they  are  the  most  important  of 
all  forms  of  igneous  agency,  since  it  is  by  movements 
such  as  these  that  continents  and  sea-bottoms,  mountains 
and  great  valleys,  have  been  formed.  Volcanoes  and 
earthquakes  occur  suddenly,  fill  the  mind  with  terror, 
and  pass  away,  leaving  behind  little  effect  on  the  config- 
uration of  the  earth  ;  but  gradual  movements  of  the 
crust,  acting  over  large  areas,  and  without  ceasing, 
through  inconceivable  ages,  have  produced  all  the  great 
inequalities  of  the  earth's  surface.  Thus  is  it  always — 
the  causes  producing  the  most  far-reaching  effects  are 
ever  those  which,  acting  slowly,  but  everywhere  and  at  all 
times,  are  scarcely  recognized  except  by  the  thoughtful 
mind. 

But  although  the  effects  of  this  form  of  igneous  agency 
are  so  important,  yet  they  are  so  obscure,  and  so  little 
has  been  accomplished  by  them  in  the  present  geological 
epoch,  that  little  is  known  of  them,  and  our  account  must 
therefore  be  brief.  It  is  their  accumulated  effects  through 
all  geological  times,  as  shown  in  the  structure  and  config- 
uration of  the  earth,  that  alone  are  conspicuous.  These 
we  shall  treat  of  in  Part  II.  In  the  meantime,  however, 
a  few  examples  of  their  action  now  will  prepare  us  for  the 
discussion  of  these  effects. 

Elevation.  —  1.  South  America. — We  have  already 
mentioned  (page  157)  that  in  1822  and  again  in  1835, 
after  severe  earthquakes,  the  southwest  coast  of  South 
America  was  elevated  several  feet  along  a  line  of  many 
hundreds  of  miles.  It  is  not  probable  that  very  much  is 
accomplished  in  this  paroxysmal  way,  but  the  fact  is 
important  as  showing  the  connection  of  earthquakes  with 
bodily  elevation  of  large  tracts.  Suppose,  then,  any 
force  beneath  tending  to  elevate  the  southern  end  of  the 
South  American  Continent,  but  resisted  by  the  stiffness 
of  the  crust ;  if  the  crust  yielded  gradually  as  the  force 


166  DYNAMICAL   GEOLOGY. 

accumulated,  only  gradual  elevation  would  take  place  ; 
but  if  the  stiffness  was  very  great,  the  yielding  might 
take  place  paroxysmally,  by  fracture,  earthquake,  and 
sudden  elevation.  The  normal  process  is,  gradual  eleva- 
tion by  gradual  yielding.  Earthquakes  are  but  occasional 
accidents  in  the  slow  march  of  these  grand  effects. 

But,  besides  these  sudden  elevations,  there  has  been 
during  an  immense  time  a  gradual  elevation  of  the  whole 
southern  part  of  'the  South  American  Continent  out  of 
the  sea.  The  evidenc'e  of  this  is  seen  in  the  old  beach- 
marks  one  above  another  to  the  height  of  1,300  feet 
above  the  sea  and  extending  along  shore  2,000  miles  on 
the  western  and  1,100  miles  along  the  eastern  coast. 
More  recently,  A.  Agassiz  has  found  on  the  same  coast 
dead  corals  of  recent  species  sticking  to  the  rocks  3,000 
feet  above  sea.  Here,  then,  we  have  continent-making 
forces  at  work  on  a  grand  scale.  It  is  not  probable  that 
the  whole  of  these  effects  was  accomplished  during  the 
present  geological  epoch,  but  they  are  the  more  interest- 
ing on  that  very  account,  since  we  here  trace  geological 
causes  directly  into  causes  now  in  operation. 

2.  Italy. — The  most  carefully  observed  example  of 
gradual  elevation  is  that  at  the  Bay  of  Baiae  near  Naples. 
Fig.  86  is  a  map  of  the  Bay  of  Baiag.  From  the  present 
shore-line  there  runs  back  a  flat  plain  of  stratified  vol- 
canic matter  sloping  gently  to  the  sea,  called  the  8tarza  ; 
this  is  terminated  by  a  perpendicular  cliff.  In  the  vicin- 
ity are  evidences  of  volcanic  action  in  the  form  of  vol- 
canic cones  and  solfataras  of  very  recent  origin.  Fig.  87 
is  a  section  of  the  same. 

Now,  there  is  abundant  proof  that  this  coast  has  slowly 
sunk  and  risen  again  at  least  twenty  feet,  and  that  this 
has  all  taken  place  certainly  since  Roman  times,  and 
probably  since  1200  A.  D.  The  evidence  is  briefly  as 
follows  :  1.  The  Starza  consists  of  stratified  material  con- 
taining recent  Mediterranean  shells.     2.  The  cliff*  which 


IGNEOUS  AGENCIES. 


167 


fcerminates   the   Starza   is  obviously   an    old    shore-cliff. 
3.  The  face  of  this  cliff  up  to  a  line  twenty  feet  above 


BozzaoU  I 
Fig.  86.— Map  of  Bay  of  Baise. 

sea-level  is  riddled  with  holes  bored  by  Uthodomi,  a  spe- 
cies of  marine-boring  shell.  4.  On  the  Starza  have  been 
found  the  remains  of  an  ancient  Eoman  temple.  When 
found,  only  the  upper  parts  of  three  fine  columns  were 
visible,  but,  by  removal  of  the  soil  twelve  feet  deep,  a 
beautiful  tessellated 
pavement  and  many 
broken  columns  were 
exposed.  The  pave- 
ment and  buried  por- 
tions of  the  columns 
were  smooth  and  well 
preserved ;  then  fol- 
lowed nine  feet  riddled  with  lithodomi,  above  which  it  was 
again  smooth.  The  uppermost  borings  were  on  the  same 
level  as  those  on  the  cliff,  and  therefore  mark  the  former 
level  of  the  sea.  Inscriptions  on  the  pavement  show  that  the 
temple  was  repaii'ed  in  the  third  century,  and  it  was  tlien, 
therefore,  above  sea-level  The  limit  of  the  borings  shows 
that  it  subsequently  sank  twenty-one  feet,  and  again  rose 
slowly  to  the  original  level,  for  the  floor  is  now  above  sea- 


87.  — Section  of  map  of  Bay  of  Baiae. 


168  DYNAMICAL  GEOLOGY, 

level.     All  this  was  done  so  quietly  that  it  was  unre- 
marked by  contemporaneous  writers. 

There  is  good  reason  to  think  that  the  whole  took 
place  between  a.  d.  1200  and  1600.  Writers  of  the  six- 
teenth century  say  that  in  1530  one  might  stand  on  the 
cliff,  b,  and  fish  in  the  sea  ;  this,  therefore,  was  during 
the  period  of  subsidence.  Now,  in  1198  a  great  earth- 
quake destroyed  Pozzuoli,  and  in  1535  Monte  Nuovo  was 
formed  by  eruption.  It  is  probable,  therefore,  that  the 
history  of  events  was  briefly  this  :  After  the  earthquake 
of  1198,  the  sinking  commenced,  and  continued  until  it 
reached  twenty-one  feet  ;  it  remained  in  this  condition 
until  the  eruption  of  1535,  when  it  began  to  rise  again. 
During  the  interval  of  subsidence,  sediments,  volcanic 
ashes,  etc.,  filled  up  the  bottom  twelve  feet  deep,  and 
protected  the  lower  part  of  the  columns,  and  only  the 
part  representing  clear  water  was  bored. 

Other  evidences  of  movements  up  or  down  are  found 
all  along  the  coasts  of  the  Mediterranean.  The  ruins  of 
the  Temple  of  the  Nymphs  are  now  in  water.  The  bridge 
of  Caligula  is  bored  several  feet  above  the  sea-level,  etc. 

3.  Sweden  and  Norway. — The  examples  thus  far  given 
are  in  volcanic  countries,  and  possibly  caused  by  volcanic 
forces  ;  but  such  movements  are  by  no  means  always  asso- 
ciated with  volcanism ;  for  example,  Scandinavia  is  re- 
markably free  from  volcanism,  and  yet  the  whole  coast, 
both  on  the  Atlantic  and  the  Baltic  side,  has  been  for  a 
long  time,  and  is  still,  rising  out  of  the  sea.  The  rate  is 
less  in  the  southern  part  and  increases  northward,  the 
average  being  about  two  to  three  feet  per  century.  That 
this  has  been  going  on  for  a  long  time  is  shown  by  old 
beach-marks  at  various  levels  up  to  six  hundred  feet  above 
sea-level,  showing  an  elevation  to  that  extent,  and  that , 
during  the  present  geological  epoch.  At  the  rate  of  two 
and  a  half  feet  per  century,  this  would  require  two  hun- 
dred and  forty  centuries,  or  twenty-four  thousand  years. 


IGNEOUS  AGENCIES. 


169 


This  is  of  course  only  an  approximate  estimate,  but  we 
may  say  with  confidence  that  for  thousands  of  years  the 
whole  of  Scandinavia,  and  perhaps  much  more,  has  been 
rising  bodily  out  of  the  ocean. 

Subsidence. — 1.  Greenland. — The  coast  of  Greenland, 
for  six  hundred  miles,  is  now  subsiding,  but  at  what  rate 
is  not  known.  The  subsidence  is  proved  by  the  fact  that 
the  houses  built  by  the  early  Norwegian  discoverers  are 
now  partially  submerged.  The  fact  is  so  well  recognized 
by  the  Eskimos  that  they  never  build  near  the  sea-level. 

2.  River  Deltas. — In  all  great  river  deltas  and  perhaps 
we  might  say  in  all  places  where  abundant  sediments  are 
accumulating,  the  earth-crust  subsides  as  if  weighted 
down  with  the  ever-increasing  load.  In  digging  or  boring 
into  the  delta  of  the  Mississippi,  the  Ganges,  or  the  Po, 
the  deposit  is  found  to  consist  of  an  alternation  of  river 
sediments  with  old  forest-grounds,  and  sometimes  peat 
several  feet  thick,  and  occasional  layers  of  limestone. 
This  is  represented  in  Fig.  88,  in  which  5  5  is  the  surface. 


Fig.  88.— Section  of  river  delta,    w,  surface  ;  rs,  river-silt ;  fg,  f orest-gronnd  j 
/,  limestone. 


with  growing  vegetation  and  accumulated  vegetable  mold, 
and  perhaps  peat.  As  we  go  down  we  pass  through  river- 
silt,  r  s,  then  an  old  submerged  forest-ground, /^,  with 
black  mold  and  stumps  in  place,  as  they  grew,  sometimes 


170  DYNAMICAL  GEOLOGY. 

with  a  considerable  layer  of  peat^  then  more  river-silt, 
with  an  occasional  layer  of  limestone,  and  so  on,  several 
times  repeated.  Such  old  forest-grounds  have  been  found 
in  the  Mississippi  delta  fifty  feet  below  sea-level,  and  in 
the  Ganges  layers  of  peat  fifty  feet  below  sea-level,  and 
fresh-water  shells  and  river-silt  near  four  hundred  feet. 
In  the  delta  of  the  Po,  peaty  layers  are  found  four  hun- 
dred feet  below  sea-level  (Lyell). 

Now,  the  only  way  possible  to  explain  these  facts  is  to 
suppose  a  slow  subsidence  on  the  one  hand  and  the  up- 
building by  sedimentation  on  the  other,  but  not  always 
absolutely  at  the  same  rate.  When  the  upbuilding  pre- 
vailed, the  area  was  reclaimed  and  overgrown  with  forest. 
When  the  subsidence  prevailed,  the  trees  were  submerged 
and  destroyed,  rotted  to  stumps  and  buried  in  sediments. 
Sometimes  the  subsidence  was  so  rapid  that  salt-water 
conditions  prevailed  and  limestones  were  formed.  Sub- 
merged forests  are  found  not  only  in  deltas,  but  also  on 
many  coast-lines,  and  are  among  the  surest  signs  of  sub- 
mergence. 

3.  Mid- Pacific  Bottom. — But  the  grandest  example  of 
subsidence,  still  in  progress,  is  undoubtedly  that  already 
discussed  under  coral  reefs.  As  already  shown,  we  have 
evidence  that  over  an  area  of  10,000,000  square  miles  in 
mid-Pacific  there  has  been,  in  comparatively  recent  geo- 
logical times,  a  subsidence  of  10,000  feet,  and  that  the 
subsidence  is  still  going  on.  Surely,  in  this  case,  we 
have  changes  now  in  progress  which  are  of  the  nature  of 
those  by  which  continents  and  sea-bottoms  were  formed. 

4.  River-beds. — Our  examples  thus  far  are  all  from 
the  coast  region.  The  phenomena  are  plainest  there, 
because  we  have  the  sea-level  as  a  term  of  comparison. 
But  in  the  interior  of  continents  we  have  river  beds  as 
indicators  of  movement.  We  have  seen  (p.  28)  that  in  a 
rising  country  rivers  cut  deeper,  while  in  a  sinking 
country  they  build  up  by  deposit. 


IGNEOUS  AGENCIES.  171 

Cause  of  Crust  Movements. 

It  is  evident  tliat  the  thing  actually  observed  is  only 
changes  in  the  relative  level  of  sea  and  land.  In  the  inte- 
rior of  continents  we  have  no  means  of  determining  such 
movements,  except  by  river  beds,  as  just  explained.  The 
cause  of  these  slow  changes  is  very  obscure  and  can  not  be 
discussed  here.*  Suffice  it  to  say  that  the  great  inequali- 
ties of  the  earth^s  crust,  such  as  continents,  ocean  basins, 
and  mountain  chains,  are  probably  due  to  the  slow  cooling, 
unequal  shrinking,  and  consequent  slight  deformation  of 
the  whole  earth,  progressive  through  all  geological  time. 

General  Retrospect, 

We  have  discussed  briefly  the  agencies  now  in  operation 
on  the  earth's  surface,  producing  structure  and  form 
under  our  eyes.  We  believe  that  similar  agencies  have 
been  at  work  through  all  time,  and  left  their  effects  in  the 
structure  and  surface  forms  which  we  actually  find.  We 
study  the  small  and  insignificant  effects  now  produced  in 
order  that  we  may  throw  light  on  those  greater  effects 
which,  accumulating  through  all  geological  times,  are  now 
embodied  in  the  earth's  structure.  We  are  now  in  a  posi- 
tion to  examine  the  actual  structure  and  forms  of  the 
earth,  and  to  interpret  them  by  the  light  of  the  previous 
discussions. 

Again  :  Of  the  agencies  which  we  have  been  discussing 
there  are  manifestly  two  groups.  Atmospheric,  aqueous, 
and  organic  agencies  constitute  the  one,  and  igneous 
agencies  the  other.  The  one  group  tends  to  reduce  the 
inequalities  of  the  surface,  and,  acting  alone,  would  event- 
ually bring  all  to  sea-level,  and  are  therefore  called  level- 
ing agencies.  The  other  originally  caused,  and  has  ever 
tended  to  increase,  the  inequalities  of  the  surface,  and, 

*  For  fuller  discussion,  see  tlie  author's  ''Elements  of  Geology,'^ 
p   131. 


172  DYNAMICAL   GEOLOOY, 

acting  alone,  would  ere  this  have  made  them  of  incredible 
dimensions,  and  are  therefore  called  elevating  agencies. 
The  state  of  the  contest  between  these  two  opposite  forces 
at  any  time,  determined  the  distribution  of  land  and  water, 
the  height  of  continents  and  mountains,  and  depth  of 
seas,  at  that  time.  The  one  group  roughhews,  the  other 
shapes,  the  forms  of  the  earth. 


PART    II. 
STRUCTURAL  GEOLOGY. 

CHAPTER  I. 

GENERAL   FORM    AND   STRUCTURE   OF  THE   EARTH. 

General  Form. 

The  general  form  of  the  earth  is  that  of  an  oblate 
spheroid  flattened  a  little  at  the  poles.  In  other  words, 
it  is  an  ellipsoid  of  revolution  about  its  minor  axis.  The 
equatorial  diameter  is  about  twenty-six  miles  greater  than 
the  polar  diameter.  This  general  form  is  taken  at  sea- 
level,  the  land-surfaces  rising  above  and  the  sea-bottoms 
sinking  below.  This  form  is  precisely  that  which  a  liquid 
globe  would  inevitably  assume  under  the  influence  of  ro- 
tation. It  has,  therefore,  been  somewhat  hastily  concluded 
that  this  general  form  is  demonstrative  evidence  of  the 
early  incandescent  liquid  condition  of  the  earth.  It  is 
certain,  however,  that  the  earth  would  have  assumed  this 
form  by  rotation,  whether  it  were  originally  liquid  or 
solid.*  Therefore,  while  it  is  almost  certain,  from  other 
considerations,  that  the  earth  was  once  liquid,  and  assumed 
its  oblate  spheroid  form  in  that  condition,  yet  this  gen- 
eral form  alone  can  not  be  regarded  as  proof  of  that  con- 
dition. 

General  Structure. — We  have  already  stated   (page 

*  This  subject  is  more  fully  explained  in  the  author's  "  Elements 
of  Geology." 

173 


174  STRUCTURAL   GEOLOGY. 

,  132)  that  the  interior  temperature  of  the  earth  increases 
1°  for  every  fifty-three  feet  in  depth,  and  that  at  this  rate 
the  fusing  temperature  of  rocks  would  be  reached  at  about 
thirty  miles  ;  aud,  finally,  that  many  have  thence  hastily 
concluded  that  the  general  structure  of  the  earth  is  that 
of  a  globe  of  fused  rock  or  lava,  covered  with  a  thin  shell 
thirty  miles  thick.  But  we  have  also  shown  there  the 
untenableness  of  this  view.  There  are  only  two  other 
views  possible,  and  now  held.  Some  hold  that  the  earth 
is  truly  solid  throughout,  excepting  reservoirs  of  liquid 
matter  forming  the  foci  of  volcanoes.  Others  hold  that 
the  earth  consists  of — 1.  A  solid  nucleus,  which  forms  its 
greatest  part ;  2.  A  solid  crust,  comparatively  thin  ;  and, 
3.  Separating  these,  a  sub-crust  layer  of  liquid  or  semi- 
liquid  matter,  if  not  universal,  at  least  over  large  areas. 
There  are  many  geological  phenomena  which  seem  to 
make  this  last  view  most  probable. 

Density  of  the  Earth. — The  mean  density  of  the 
earth,  taken  as  a  whole,  is  5.6.  The  density  of  the  crust 
is  about  2.5.  Therefore  the  density  of  the  central  parts 
must  be  very  much  greater  than  5.6.  It  is  probably  not 
less  than  15  to  16.  This  greater  interior  density  is  due 
partly  to  a  difference  of  material  (the  denser  settling 
toward  the  center,  Avhile  tlie  earth  was  still  in  a  fused  con- 
dition), and  partly  to  condensation  hy  pressure. 

Crust  of  the  Earth. — The  surface  portion  of  the 
earth  differs  in  many  respects  from  the  interior,  and  is, 
therefore,  properly  called  a  crust:  1.  It  is  certainly  a 
lighter  portion  covering  a  denser  interior.  2.  It  is  a  cooler 
portion,  covering  an  incandescent  interior.  3.  It  is,  as 
we  shall  see  hereafter,  a  stratified  portion  covering  an 
unstratified  interior.  4.  It  is  probably  an  oxidized  por- 
tion covering  an  unoxidized  or  less  oxidized  interior  (for 
oxidation  comes  by  contact  with  air  and  water).  5.  It  is 
probably  a  solid  shell  covering  a  liquid  or  semi-liquid  sub- 
crust  layer.     It  is  this  idea  of  a  solid  shell  covering  a 


FORM  AND  STRUCTURE  OF  THE  EARTH.       1^5 

liquid  which  gave  origin  to  the  term  crust  j  but  the  word 
is  now  used  only  to  signify  the  superficial  portions  of  the 
earth,  subject  to  human  observation,  without  any  impli- 
cation as  to  the  interior  condition. 

Means  of  Geological  Observation. — As  thus  defined, 
the  crust  is  estimated  at  from  ten  to  twenty  miles  in 
thickness.  The  manner  in  which  we  get  a  knowledge  of 
tlie  earth  to  that  depth,  or  the  means  of  geological  obser- 
vation, are — 1.  By  mines  and  artesian  wells.  These  pene- 
trate 4,000  or  5,000  feet.  2.  Canons  and  ravines.  These 
give  sections  of  6,000  or  7,000  feet.    3.   Volcanic  ejections. 


Fig.  89. 


These  bring  up  matter  from  unknown  but  certainly  still 
greater  depth.  But  the  most  common  and  effective  means 
of  observation  is  furnished  by — 4.  Foldings  of  the  crust, 
and  subsequent  erosion.  In  the  section  (Fig.  89)  in  which 
5  5  is  the  present  surface,  we  represent  one  of  the  com- 
monest of  all  geological  phenomena.  It  is  seen  that 
from  the  point  a  the  strata  are  repeated  on  the  two  sides. 
The  dotted  lines  show  how  much  has  been  cut  away,  and 
what  depth  of  strata  has  been  exposed  to  view.  In  this 
way,  in  very  many  places,  the  character  of  the  rocks  ten 
or  more  miles  deep  is  revealed. 

Our  direct  observation  is  absolutely  confined  to  this 
superficial  portion.  "We  can  only  speculate  about  what  is 
beneath.     It  would  seem,  at  first  sight,  that  this  is  an 


176  STRUCTURAL   GEOLOGY. 

insignificant  portion  of  the  earth  upon  which  to  found  a 
science  of  the  earth.  But  it  must  be  remembered  that  on 
this  superficial  portion  has  been  enacted,  and  in  its  struc- 
ture has  been  recorded,  the  whole  history  of  the  earth. 

General  Surface  Configuration  of  tlie  Crust. — The 
crust  of  the  earth  is  diversified  by  greater  and  smaller 
features.  The  greater  features  are  due  to  interior  or 
.elevating,  the  lesser  to  exterior  or  leveling  agencies.  Under 
the  former  head  come  those  greatest  features,  constitut- 
ing continental  surf  aces,  and  oceanic  bottoms,  and  those 
next  greatest,  viz.,  mountain-chains  and  great  valleys^ 
Under  the  latter  come  all  those  peaks  and  ridges,  valleys 
and  ravines,  which  have  been  produced  by  subsequent 
erosion. 

The  mean  height  above  the  sea-level  of  the  continents 
is  about  1,200  to  1,300  feet,  or  less  than  one  fourth  mile, 
and  the  mean  depth  of  the  ocean-bottoms  below  the  same 
level  is  about  15,000  or  1G,000  feet,  or  nearly  three  miles. 
The  ocean-surface  being  nearly  three  times  as  great  as  the 
land-surface,  it  is  evident  that,  if  the  inequalities  of  the 
crust-surface  were  removed,  there  is  water  enough  to 
cover  the  whole  earth  more  than  two  miles  deep. 

General  Laws  of  Continental  Form. — There  are 
certain  general  laws  of  continental  form  which  have  a 
bearing  on  the  question  of  the  origin  of  continents,  and 
which,  therefore,  must  be  briefly  mentioned. 

1.  Continents  consist  essentially  of  Interior 
Basins,  with  Coast-Chain  Rims. — The  interior  basins 
are  drained  by  the  great  rivers  of  the  world.  This  typical 
structure  is  well  shown  in  America,  North  and  South,  in 
Australia,  and  in  Africa.  For  example,  in  North  America 
we  have  the  great  interior  basin  drained  by  the  Mississippi 
River,  bordered  on  the  Atlantic  side  by  the  Appalachian, 
and  on  the  Pacific  side  by  the  great  Rocky  Mountain  sys- 
tem or  American  Cordilleras,  consisting  of  many  ranges, 
of  which  Colorado,  Wahsatch,  and  the  Sierra  and  Coast 


FORM  AND  STRUCTURE  OF  THE  EARTH.       177 

Eange  of  California  are  the  most  notable  (Fig.  90,  a). 
South  America  has  the  Andes  on  one  coast,  the  Brazilian 
mountains  on  the  other,  and  the  great  interior  basin 
drained  by  the  Amazon,  La  Plata,  and  Orinoco  Rivers 
(Fig.  90,  h).  Similarly,  the  great  basin  of  Africa  is 
drained  by  the  Nile,  Niger,  Congo,  and  Zambesi  Rivers. 


Basin.       Plateau. 


Plains. 


East  and  west  section  of  North  American  Continent :  cr,  coast  range  ;  SJ^  San  Joa» 
quin  plain  ;  S,  Sierra  ;  w,  Wahsatcli ;  c,  Colorado  range ;  Ap,  Appalachian. 


Andes. 

East  and  west  section  across  South  America. 


Brazil'^  m^ 


WmmmmnmimmmmmnMiimimmmmmiL 


Fig.  90. 


Ei-ist  iiiul  v>esL  (section  across  Australia. 
-Sections  across  North  and  South  America  and  Australia. 


Australia  is  also  a  fine  example,  as  shown  in  Fig.  90,  c, 
Europe  and  Asia  have  similar  structure,  but  less  perfect. 
This  continent  is'  elongated  east  and  west,  and  therefore 
the  section  must  be  north  and  south. 

2.  The  Greater  Range  faces  the  Greater  Ocean. — 
In  America,  the  North  American  Cordilleras  and  the 
Andes  face  the  Pacific,  while  the  Appalachian  and  the 
Brazilian  mountains  face  the  Atlantic.  In  Africa  and 
Australia,  on  the  contrary,  the  east  range  faces  the 
greater  ocean,  and  is  the  greater. 

3.  The  greater  chains  are  usually  the  most  complex  and 
crumpled  in  structure,  and  give  evidence  of  greatest  vol- 
canic activity  in  the  present  or  in  the  past. 

Le  Contb,  Geol.  12 


178  STBUCTURAL   GEOLOGY. 

4.  Continents  and  ocean-bottoms  have  not,  as  some 
imagine,  frequently  changed  places.  On  the  contrary, 
the  places  of  continents  have  been  indicated  and  their 
outlines  sketched  out  from  the  beginning,  and  their  forms 
have  been  gradually  developed,  though  with  many  oscil- 
lations, throughout  all  geological  times. 

The  origin  of  continents  and  ocean-bottoms  is 
very  obscure,  but  it  is  probably  in  some  way  connected 
with  the  unequal  contraction  and  therefore  deformation  of 
the  spheroidal  form  of  the  earth,  by  slow  cooling  from  a 
former  incandescent  condition.  In  such  an  irregular  or 
deformed  spheroid,  of  course,  the  water  would  collect  in 
the  hollows,  and  the  protuberances  would  become  conti- 
nents.    The  origin  of  mountains  we  discuss  further  on. 

Rocks, 

Definition  of  Kock. — The  term  roch  is  used  in  popu- 
lar language  to  designate  any  substance  of  stony  hard- 
ness. Not  so  in  geology.  Any  substance  constituting  a 
portion  of  the  earth^s  crust,  whether  it  be  hard  or  soft, 
is  called  a  roch.  No  distinction  based  on  hardness  alone 
is  of  any  value.  The  same  sandy  bed  may  be  found  in 
one  place  hard  enough  for  building-stone,  and  in  an- 
other soft  enough  to  be  spaded.  The  same  clay  stratum 
may  sometimes  be  traced  from  a  condition  of  slaty  hard- 
ness in  one  place  to  good  brick-earth  in  another  ;  the  same 
bed  of  lime  from  marble  into  chalk,  and  the  same  volcanic 
eruption  from  stony  lava  into  a  bed  of  volcanic  ashes. 

Classes  of  Bocks. — Eocks  are  divided,  according  to 
their  structure  and  origin,  into  two  principal  kinds,  viz., 
stratified  and  unstratified.  Stratified  rocks  are  more  or 
less  consolidated  sediments,  and  are  therefore  aqueous  in 
origin  and  earthy  in  structure.  Unstratified  rocks  have 
been  more  or  less  fused,  and  therefore  are  igneous  in 
origin  and  either  crystalline  or  glassy  in  structure. 


CHAPTER  II. 


STRATIFIED   ROCKS. 


Skctiok  I. — Their  Structure  an^d  Positiom". 


Let  any  one  examine  the  rocks  of  a  quarry  of  limestone 
or  sandstone,  and  lie  will  find  that  the  stone  lies  in  regu- 
lar beds.  In  some  places  these  beds  will  lie  level  (Fig. 
91),  in  other  places  they  may  be  inclined  (Fig.  92).  For 
example,  throughout  the  valley  of  the  Mississippi  they 
are  usually  level,  while  in  mountain-regions  they  are 
usually  inclined.  The  next  most  conspicuous  structure 
will  probably  be  the  cross-divisions  called  joints,  by 
which  the  beds  are 
broken  into  separ- 
able blocks.  Thiese 
are  found  in  all 
rocks,  are  not  char- 
acteristic of  strati- 
fied rocks,  and 
therefore  we  say 
nothing  more  about 
them  now.  On  ex- 
amining a  little 
more  closely,  the 
beds  will  be  seen  to 
be  subdivided  by 
faint  lines  similiar  to  those  observed  in  a  section  of  sedi- 
ments, and  known  to  be  produced  by  the  sorting  power 

179 


Figs.  91,  92.— Sections  of  horizontal  and  inclined 
strata :  s,  soil ;  ss,  sandstone  ;  «A,  shale ;  Z«», 
limestone. 


180  STRUCTURAL   OEOLOOY. 

of  water  (page  27).  In  a  word,  the  mass  exposed  on  a 
cliff  or  in  a  quarry,  or  any  large  section  of  stratified  rock, 
is  seen  to  be  divided  by  parallel  planes  into  thick  beds  of 
different  kinds  of  materials,  as  sandstone,  limestone,  etc., 
and  each  of  these,  probably,  into  thinner  beds,  differing 
perhaps  in  grain  or  color,  and  finally  these  again  into  thin 
sheets,  produced  by  the  sorting  of  material.  Now,  the 
larger  beds  are  called  strata,  the  subdivisions  of  different 
color  or  grain,  layers,  and  the  lines  of  sorted  materials  are 
lamincB.  These  terms  are  loosely  used,  but  always  in  the 
order  mentioned,  and  the  word  lamina  is  always  used  to 
signify  the  marks  of  water-sorting.  Now,  the  structure 
we  have  described  is  called  stratification,  and  such  rocks 
stratified  rocJcs. 

Extent  and  Thickness. — Stratified  rocks  cover  at 
least  nine  tenths  of  the  land-surface,  and  even  where  they 
do  not  occur  it  is  only  because  they  have  been  removed 
by  erosion  or  else  covered  by  igneous  rocks.  Since,  as  we 
shall  see  presently,  stratified  rocks  were  formed  at  the 
bottom  of  the  water,  it  is  evident  that  there  is  no  portion 
of  the  earth  which  has  not  been  at  some  time  covered  by 
the  sea.  The  extreme  thickness  of  these  rocks  is  proba- 
bly ten  to  twenty  miles  ;  the  average  thickness  is  certainly 
several  thousand  feet. 

Principal  Kinds. — As  defined  above,  stratified  rocks 
fall  naturally  into  three  great  groups  :  1.  Arenaceous  or 
sand-rocks ;  2.  Argillaceous  or  clay-rocks  ;  and,  3.  Cal- 
careous or  lime-rocks.  These  may  be  either  in  a  soft  or 
in  a  stony  condition. 

The  sand-rochs,  in  their  soft  or  incoherent  condition, 
are  beds  of  sand,  gravel,  and  pebbles  or  shingle.  In  their 
coherent  or  stony  condition  they  are  sandstones,  grits, 
and  conglomerates.  Breccias  differ  from  conglomerates 
only  in  having  the  fragments  angular  instead  of  rounded. 
They  consist  of  rubble,  instead  of  pebbles.,  cemented 
together. 


STRATIFIED  ROCKS.  181 

The  clay-rocks,  in  their  incoherent  condition,  are  beds 
of  clay,  brick-earth,  mud,  and  ooze.  In  their  coherent 
condition  they  are  the  same  cemented  into  shales,  or,  still 
harder,  into  slates. 

Lime-rocks,  in  an  incoherent  condition,  are  lime-muds, 
such  as  exist  now  in  coral  lagoons,  or  in  the  deep  sea  (glo- 
bigerina  ooze,  page  117)  ;  in  a  slightly  consolidated  con- 
dition they  are  chalks,  and  in  a  stony  condition  they  are 
limestones,  marbles,  and  travertines. 

These  different  kinds  may  each  produce  varieties  of 
different  color  and  grain.  They  also  pass  by  mixture 
insensibly  into  each  other,  and  thus  form  infinite  varie- 
ties. Thus  we  may  have  an  argillaceous  or  calcareous 
sandstone  or  calcareous  shale,  etc. 

All  that  need  further  be  said  on  the  subject  of  the 
origin  of  stratified  rocks  is  best  thrown  into  a  series  of 
propositions,  very  simple  and  yet  underlying  all  geologi- 
cal reasonings  : 

1.  Stratified  Rocks  are  more  or  less  Consolidated 
Sediments. — This  has  been  thus  far  assumed.  We  wish 
now  to  direct  the  pupil  to  the  observation  of  the  evidence  : 
a.  Every  gradation  may  be  traced  between  muds,  clays, 
and  sands,  which  we  know  were  deposited  in  water  ;  and 
shales  and  sandstones,  which  we  find  forming  the  strata 
of  mountains.  5.  In  many  cases  we  may  see  the  process 
of  hardening  going  on. under  our  eyes.  For  example,  at 
the  mouths  of  rivers  carrying  lime  in  solution,  like  the 
Rhine,  the  river-silts  are  consolidated  into  calcareous 
shales.  On  the  shores  of  coral  reefs  we  find  coral  mud, 
coral  sand,  and  coral  breccia  consolidated  into  peculiar 
limestones  (page  108).  c.  Close  examination  of  many 
rocks,  especially  sandstones  and  shales,  clearly  shows  the 
sorting  of  material  (water-sorting)  along  the  lines  of  lam- 
ination, d.  As  shells  and  skeletons  of  animals  are  now 
imbedded  in  muds  of  rivers,  lakes,  and  seas,  so  fossils  are 
found  in  stratified  rocks,     e.  Other  marks,  which  occur 


182  ■  STRUCTURAL  QEOLOQY, 

in  recent  sediments,  such  as  ripple-marks,  rain-prints, 
sun-cracks,  foot-prints  of  animals,  etc.,  are  also  found  in 
the  hardest  stratified  rocks.  In  a  word,  it  may  be  said 
that  every  mark  or  peculiarity  which  has  'been  observed  in 
recent  sediments  has  been  found  also  in  stratified  rocks. 

We  may  assume,  then,  as  certain  that  stratified  rocks 
are  sediments  formed  originally  at  the  bottom  of  seas, 
lakes,  rivers,  etc.,  and  that  when  we  find  them  far  in  the 
interior  of  continents  and  high  up  the  slopes  of  mountains 
we  have  indubitable  evidence  of  great  changes  of  level. 

Stratified  rocks  are  all  deposits  in  water.  Sandstones 
•and  shales  are  the  debris  of  erosion,  and  are  therefore 
mechanical  deposits;  and  these  rocks  are  often  called 
fragmental  rocks,  because  they  are  made  up  of  the  frag- 
ments of  previous  rocks.  Limestones,  on  the  other  hand, 
are  either  organic  or  chemical  deposits.  Again,  sand- 
stones, grits,  and  conglomerates  are  formed  by  violent 
action,  and  they  indicate  either  rapid  currents  or  exposed 
shores  ;  shales  indicate  quiet  seas  or  bays ;  limestones^ 
open  seas. 

We  have  already  seen  (page  27  et  seq.)  that  sediments 
are  transported  soils,  and  (page  10)  that  soils  are  disinte- 
grated rocks.  Now,  we  see  that  stratified  rocks  are  con- 
solidated sediments.  We  have  here  an  example  of  a  per- 
petually recurring  cycle  of  changes  :  rocks  are  decomposed 
into  soils,  soils  are  carried  and  deposited  as  sediments, 
sediments  are  again  consolidated  into  rocks,  to  be  raised 
into  land-surfaces,  and  again  disintegrated  into  soils — and 
so  the  cycle  goes  round. 

The  cause  of  consolidation  is  sometimes  only  the 
pressure  of  great  thickness  of  sediment ;  sometimes  the 
same,  aided  by  gentle  heat ;  sometimes  there  is  a  distinct 
cementing  substance,  the  most  common  being  lime  car- 
bonate and  silica.  When  there  is  a  cementing  substance, 
the  process  is  often  rapid,  and  may  be  observed ;  as,  for 
example,  in  the  formation  of  coral  rock.     But  in  other 


STRATIFIED  ROCKS. 


183 


cases  the  process  is  very  slow,  and  therefore  the  newer  rocks 
are  often,  though  not  alwaySp  imperfectly  consolidated. 

2.  Stratified.  Rocks  have  been  gradually  deposited. 
— By  this  we  mean  that  they  have  not  been  formed  at 
once,  as  some  of  the  older  geologists  imagined,  but  by  the 
regular  operation  of  causes  similar  to  those  now  accumu- 
lating sediments.  The  slowness  was  sometimes  extreme. 
For  example  :  a.  We  have  strata  in  which  the  laminae  are 
as  thin  as  paper,  and  yet  each  one  represents  recurring 
conditions,  as  ebb  and  flow  of  tide,  or  flood  and  low  water 
of  rivers,  h.  In  some  cases  we  have  a  shell  attached  to 
the  inside  of  another  shell  (Fig.  93),  in  such  wise  that  the 
latter  shell  must  have  been  dead  before  the  former 
attached  itself.     In  such  cases 

a  half  or  quarter  inch  thickness 
of  rock  represents  the  whole 
life  of  the  second  shell,  c.  We 
have  seen  that  some  limestones 
are  made  up  of  the  accumulated 
remains  of  successive  genera- 
tions of  microscopic  shells 
(page  115).  Every  inch  thick- 
ness of  such  deposit  must  rep- 
resent a  long  period  of  time. 
And  yet  such  deposits  are  often 
hundreds  or  even  thousands  of 
feet  in  thickness.  These  are, 
however,  extreme  cases  of  slow- 
ness. As  a  general  rule,  coarser 
materials  are  deposited  more  rapidly  than  finer — e.  g., 
sands  than  clays  and  limestone,  but  all  by  regular  opera- 
tion of  causes  ;  and  therefore,  making  due  allowance  for 
the  nature  of  the  materials,  thickness  is  a  rough  measure 
of  time. 

3.  Stratified  Rocks  were  originally  horizontal  at 
the  Bottom  of  the  Water. — This  is  a  necessary  conse- 


FiG.  98.— Serpulae  on  interior  of  a 
shell. 


184 


STRUCTURAL  GEOLOGY. 


quence  of  the  manner  in  which  they  were  f  ormedo  There- 
fore, when  we  find  them  in  other  positions  and  at  other 
levels,  we  conclude  that  they  have  come  so  by  subsequent 
change. 

We  must  not  imagine,  however,  that  the  planes  between 
the  strata  were  ever  absolutely  horizontal.  Strata  must 
not  be  likened  to  continuous,  even  sheets,  but  rather  to 
extensive  cakes,  thickest  in  the  middle  and  thinning  on 
the  margins  and  there  interlapping  with  other  strata  or 
cakes  (Fig  94),     Coarse  materials,  like  sandstones  and 


Fig.  94.— Diagram  showing  thinning  out  of  beds  i 
b,  limestones. 


a,  sandstones  and  conglomerates; 


grits,  are  more  local,  and  thin  out  more  rapidly,  while  fine 
materials,  like  clays,  are  often  very  widely  continuous. 
This  thinning  out  of  strata,  however,  does  not  interfere 
seriously  with  their  appearance  of  evenness  at  any  point 
of  observation. 

Another  more  important  apparent  exception  to  original 
horizontality  is  what  is  called  cross-lamination  or  false- 
deciding  (Fig.  95).     These  are  liable  to  be  mistaken  for 


Fig.  95.— Section  on  Mississippi  Central  Railroad  at  Oxford  (after  Hilgard) ;  oblique 
lamination. 


STRATIFIED  ROCKS.  185 

tilted  strata.  But  it  will  be  observed  that  it  is  the  laminae, 
and  not  the  strata,  which  are  inclined.  And,  moreover, 
their  extreme  irregularity  is  sufficient  to  distinguish  them 
from  true  inclined  strata.  They  seem  always  to  be  pro- 
duced by  deposit  from  rapid,,  shifting,  overloaded  currents, 
and  are,  therefore,  common  in  river-deposits. 

After  explaining  these  apparent  exceptions,  we  come 
back  with  still  more  confidence  to  the  proposition  that 
stratified  rocks  were  originally  soft  sediments  in  a  hori- 
zontal position  at  the  bottom  of  seas,  lakes,  etc.  But  we 
usually  find  them  noio  in  an  entirely  different  condition 
and  position.  We  indeed  find  them  sometimes  soft,  but 
more  commonly  stony  ;  sometimes,  indeed,  still  horizon- 
tal, though  raised  above  the  sea  and  in  the  interior  of 
continents,  but  more  commonly  more  or  less  tilted  ;  some- 
times, especially  in  mountain-regions,  not  only  tilted,  but 
folded,  crushed,  contorted,  broken,  and  dislocated  in  the 
most  complex  manner,  so  that  it  is  difficult  to  make  out 
their  natural  order.  Sometimes  the  contortion  is  in  the 
lamincB,  so  that  it  can  be  seen  in  a  hand-specimen  (Fig. 
9G).     Sometimes  a  series  of  strata  are  folded  together. 


Fio.  96.— Crumpled  laminae.    (After  Geikie.) 

such  as  may  be  seen  at  one  view  on  an  expooCvl  cliff  (Fig. 
97).     Sometimes  the  strata  composing  the  crust  of  the 


186 


STRUCTURAL   GEOLOGY, 


Fig.  97.— Contorted  strata.     (From  Logan.) 

earth,  several  thousand  feet  thick,  are  folded  all  together 
so  that  their  foldings  form  great  mountain-ridges,  and 
can  only  be  made  out  by  extensive  surveys  (Fig  98).     As 


Pig.  98.— Section  of  Appalachian  chain. 


might  be  expected,  the  strata  by  such  violent  movements 
are  usually  broken  and  dislocated,  and  always,  as  seen  in 
Pigs.  97  and  99,  large  portions  of  their  upper  parts  have 


Fig.  99. 


been  carried  away  by  erosion,  leaving  their  edges  exposed 
on  the  surface.  Such  exposure  of  strata  on  the  surface  is 
called  outcrop. 


Fi©.  100, 


STRATIFIED  ROCKS. 


187 


This  important  subject  must  be  taken  up  with  some 
detail,  and  for  this  purpose  it  becomes  necessary  to  define 
some  common  geological  terms. 

Dip  and  Strike.— The  angle  of  inclination  of  strata 
^with  the  horizon  is  called  the  dip.  There  are  always  two 
'elements  to  be  considered ;  viz.,  direction  and  amount. 
Thus  a  stratum  may  dip  northward  30°.  The  angle  of  dip 
varies  from  0  to  90° — i.  e.,  from  horizontality  to  verticality. 
Sometimes  strata  are  even  pushed  over  beyond  the  vertical 
— such  are  called  overturn-dips  (Fig.  99).  Examples  are 
found  in  all  great  mountain-chains,  especially  in  the  Alps. 
When  strata  dip  regularly,  their  thickness  may  he  easily 
estimated.  For  example,  in  walking  from  a  to  h  (Fig. 
100),  we  pass  over  strata  whose  thickness  is  5  c  (=  a  J  .  sin 
t  a  c).  The  dip  may  be  accurately  determined  by  means 
of  a  clinometer  (Fig.  101). 


Fig.  101.— Clinometer. 

The  direction  of  strata,  or  their  line  of  intersection 
with  a  horizontal  plane,  is  called  the  strike.  It  is  always 
at  right  angles  to  the  dip.  If  the  dip  is  so  many  degrees 
north  or  south,  the  strike  will  he  east  and  west.  If  the 
surface  of  the  ground  is  level,  the  strike  will  be  the  same 
as  the  outcrop,  or  appearance  on  the  surface,  of  the 
strata  ;  but  this  is  seldom  the  case.  If  the  strata  are 
plane,  the  strike  will  be  a  straight  line.  If  the  strata  are 
folded,  the  strike  may  be  very  sinuous  (Fig.  107).  In  a 
map  view  of  strata,  the  dip  and  strike  are  represented  by 
the  sign  1,  in  which  the  heavy  line  represents  the  strike. 


188 


STRUCTURAL   GEOLOGY. 


and  the  perpendicular  the  dip  (Fig.  105).     The  perpen- 
dicular is  made  shorter,  as  the  dip  is  at  a  higher  angle. 

Anticline  and  Syncline. — When  a  series  of  strata  dip 
in  one  direction  in  one  place,  the  same  series  will  usually 
be  found  to  dip  in  a  contrary  direction  in  another  place. 
In  other  words,  strata  are  usually  disturbed  by  lateral 
pressure,  which  throws  them  into  folds,  sometimes  wide 
and  gentle,  like  undulations,  sometimes  closely  appressed. 
Thus  strata  usually  occur  in  alternate  saddles  and  troughs 
(Figs.  102,  103).  The  saddles  are  called  anticlines,  the 
troughs  synclines.     An  anticlinal  axis,  then,  may  be  de- 


FiG.  102. 

fined  as  a  line  on  either  side  of  which  the  strata  repeat 
one  another,  dipping  in  opposite  directions,  away  from 
the  axis.  A  synclinal  axis  is  a  line  on  either  side  of 
which  the  strata  repeat  each  other,  dipping  in  opposite 
directions,  but  toward  the  axis.  In  Figs.  102  and  104,  a 
is  an  anticline,  and  s  a  syncline. 

In  anticlines  the  strata  lie  in  saddles  and  in  synclines 
in  troughs,  but  the  surface  configuration  of  the  ground 
may  or  may  not  correspond.  Sometimes  the  ground  is 
comparatively  level,  though  the  foldings  are  strongly 
marked  (Fig.  102).  Sometimes  the  anticlines  are  ridges, 
and  the  synclines  valleys  (Fig.  103),  and  sometimes  the 


Fig.  103. 


STRATIFIED  ROCKS. 


189 


reverse  (Fig.   104).     In   gently  folded   strata   it  is  very 
common  to  find  the  configuration  reversed  on  the  surface. 


Fig.  104. 


i.  e.,  synclinal  ridges  and  anticlinal  valleys.     Examples 
of  these  are  given  on  page  248. 

Folded  strata,  which  are  tilted   only  hy  folding,  will 
outcrop  on  level  ground  in  parallel  bands,  as  in  Fig.  105, 


It 

II 

II 

1 

" 

,,^^__ 

_^ 

II  j 
II  tj 

l|;; 

II 

l* 

II 

II 

K 

II 

;; 

II 

" 

ill 

II 

II    1 

» 

II 

III 

A 

1 

" 

\\    }/j| 

in " 

Fig.  105. 


which  is  a  map  view  of  Fig.  102.     But  if  the  whole  be 
again  lilted  in  a  direction  at  right  angles  to  the  folds. 


Fig.  106.— Section  of  undulating  strata, 
■t 

then  the  map  of  outcrop  will  be  sinuous.  Fig.  106  is  a 
section  of  folded  strata  thus  tilted,  and  Fig.  107  is  a  map 
of  the  same.  The  section  is  along  the  line  C  D.  Exam- 
ination of  the  signs  of  dip  will  explain  the  map. 


190 


STRUCTURAL  GEOLOGY. 


Fig.  107.— Plan  of  undulating  strata. 

We  have  spoken  of  folded  strata  and  the  way  in  which 
they  outcrop  ;  but  in  a  survey  the  process  is  reversed,  i.  e., 
it  is  the  outcrop  which  is  observed,  and  from  this  we  con- 
struct the  section.  Kow,  when  we  remember  the  complex 
folding,  then  the  tilting  after  folding,  then  the  displace- 
ment by  fractures,  and  then,  worst  of  all,  the  covering  of 
the  whole  deeply  with  soil,  leaving  exposed  only  patches 
here  and  there,  we  can  easily  see  how  difficult  a  problem 
it  often  is  to  construct  a  section  of  the  stratified  rocks  of 
a  country.  If  the  strata  be  exposed  on  a  cliff  or  a  cafi on- 
side,  there  is  little  difficulty,  but,  in  the  absence  of  such, 
the  geologist  takes  advantage  of  every  exposed  patch, 
examines  every  gulch  or  stream-bed,  every  quarry  or 
railroad-cutting,  and  thus  constructs  an  ideal  section. 

Conformity  and  Unconformity. — We  have  just  seen 
that  the  strata  composing  the  country  rock  of  a  land- 
surface  are  usually  tilted  and  crumpled  and  always  eroded, 
so  that  their  edges  are  exposed  (see  Figs.  106, 107).  But 
we  have  also  seen  (pages  164-170)  that  in  some  places 
land-surfaces  are  now  sinking  beneath  the  sea,  and  in 
others  sea-bottoms  are  rising  to  become  land-surfaces. 
The  same  is  true  for  all  geological  epochs.  Now,  suppose 
at  any  time  an  eroded  land-surface  sank  below  sea-level 
so  that  sediments  were  deposited  on  the  eroded  edges  and 
filling  the  erosion-hollows  of  the  strata,  and  finally  the 


STRATIFIED  ROCKS. 


191 


ynole  was  again  raised  above  sea  and  exposed  to  the  in- 
iDection  of  the  geologist ;  the  phenomena  which  would 


Fig.  108,— Some  cases  of  unconformity. 


be  observed  are  represented  by  Fig.  108,  A,  B,  C.  This 
is  what  is  called  iinconformity .  More  commonly  in  such 
cases  there  is  a  want  of  parallelism  between  the  two  series 
of  strata,  as  in  Fig.  108,  A,  B.  But  this  is  not  necessary. 
Fig.  108,  C,  represents  unconformity  no  less  than  A  and  B, 
In  the  one  case  the  strata  were  raised  into  land-surface 
and  at  the  same  time  folded  and  tilted,  and  then  eroded ; 
in  the  other  case,  they  were  raised  and  eroded  without 
folding  or  tilting.  Sometimes  the  second  raising  is  also 
attended  with  tilting,  in  which  case  both  series  are  tilted, 
but  in  different  degrees,  as  in  B. 

Definition. — After  this  explanation,  we  are  prepared 
to  define.  When  a  series  of  strata  are  parallel,  as  if 
formed  continuously  under  similar  conditions,  they  are 


192  STRUCTUnAL  GEOLOGY. 

Baid  to  be  conformable.  But  if  two  series  are  discontinu- 
ous— i.  e.,  separated  by  an  erosion-surface  or  old  land-sur- 
face, and  therefore  formed  at  different  times  and  under 
diiferent  conditions — they  are  said  to  be  unconformable. 
In  all  the  figures  the  strata  of  the  lower  series  are  con= 
formable  throughout,  and  so  are  also  those  of  the  upper, 
but  the  two  series  are  unconformable  with  each  other,  the 
line  of  unconformity  being  an  old  eroded  land-surface. 

Even  so  simple  sections  as  Fig.  108,  one  of  the  com- 
monest observed,  record  many  interesting  events  in  the 
history  of  the  earth,  viz.  :  1.  A  long  period  of  quiet, 
during  which  the  first  series  of  strata  was  deposited. 
2.  A  period  of  commotion,  during  which  the  sea-bottom 
here  was  elevated  into  land,  and  perhaps  the  strata  crum- 
pled. 3.  A  long  period  during  which  it  remained  land- 
surface  and  was  deeply  eroded  and  the  strata-edges  exposed, 
4.  Another  period  of  commotion,  during  which  it  sank 
again  and  became  sea-bottom.  5.  Another  long  period 
of  quiet,  during  which  the  second  series  of  strata  was  de- 
posited ;  and,  6.  Still  another  period  of  movement,  by 
which  the  whole  was  finally  raised  and  became  thus  sub- 
ject to  the  inspection  of  the  geologist. 

The  following  diagrams  (Fig.  109)  represent  the  man- 
ner in  which  the  phenomena  may  be  supposed  to  have 
occurred.  In  A,  we  have  thick  sediments,  Sd,  accumu- 
lated on  an  off-shore  sea-bottom.  In  B,  the  same  have 
been  elevated  into  land,  and  crumpled.  In  (7,  they  have 
been  eroded  and  their  edges  exposed.  In  D,  they  have 
again  subsided  beneath  the  sea,  and  received  sediments, 
Sd,  on  their  eroded  edges. 

Since  geological  history  is  mainly  recorded  in  stratified 
rocks,  and  since,  while  a  place  is  land-surface  and  being 
eroded,  there  can  -be  no  strata  formed  there,  it  is  evident 
that  a  lino  of  unconformity  always  indicates  a  period  of 
which  there  is  no  record  at  that  place,  although  the  record 
may  be  found  elsewhere.     Unconformity,  therefore,  al- 


STRATIFIED  ROCKS. 


193 


ways  represents  a  gap  in  the  record — a  lost  interval  of 
time— which  may  be  very  long,  viz.,  the  whole  time  dur- 
ing which  the  erosion  Avas  going  on. 


Fig.  109.— In  all  :  Z,  laud  ;  II,  sea-level  ;  Sh,  shore-line  ;  Sh,  sea-bottom  ; 
sediments. 


A  group  of  conformable  strata  usually  form  a  geologi- 
cal formation,  and  a  line  of  unconformity  usually  sepa- 
rates two  different  geological  formations.  The  division 
of  the  strata  into  formations,  however,  is  based  also  on 
other  characters,  viz.,  the  contained  fossils.  The  subject 
will  "be  taken  up  again  under  that  head. 

Cleavage  Structure. 

Stratification  is  an  original  structure,  i.  e.,  impressed 
at  the  time  of  deposit  of  sediments.  Cleavage  is  a  super- 
induced  or   subsequent   structure,    but   it   so    simulates 

Lb  Conte,  Gbol.  13 


194  STRUCTURAL   OEOLOOY. 

stratification  that  it  seems  best  to  take  it  up  here.  It  is 
found  in  many  kinds  of  rocks,  but  most  perfectly  in 
slates,  and  is  therefore  often  called  slaty  cleavage. 

Definition. — Cleavage  is  easy  splitting  in  certain  di- 
rections. There  are  many  kinds  of  cleavage  due  to  dif- 
erent  causes.  For  example,  many  crystals  split  perfectly 
in  certain  directions.  This  is  called  crystalline  cleavage, 
and  is  due  to  molecular  arrangement.  Certain  stratified 
sands  split  easily  into  broad  flag-stones  in  the  direction 
of  the  laminae.  This  is  lamination  cleavage,  and  is  due  to 
the  arrangement  of  the  grains  by  the  sorting  'power  of 
water.  Again,  wood  splits  easily  in  the  direction  of  the 
silver  grain.  This  wood-cleavage  is  due  to  the  arrange- 
ment of  the  wood-cells. 

Slaty  Cleavage. — Now,  there  is  also  an  easy  splitting 
of  rocks  in  definite  directions,  which  occurs  on  an  im- 
mense scale,  and  in  certain  slates  is  a  very  marked  struc° 
ture.  The  direction  of  cleavage  is  usually  vertical  or 
highly  inclined.  Whole  mountains  are  thus  cleavable 
from  top  to  bottom,  and  rocks  over  thousands  of  square 
miles  are  often  made  up  of  such  thin  sheets.  It  is  by 
splitting  along  these  lines  of  easy  fracture  that  roofing- 
slates,  ciphering-slates,  and  blackboard-slates  are  made. 

On  casual  examination  of  strata  the  cleavage-planes  are 
liable  to  be  mistaken  for  fine  laminm,  and  we  are  apt  to 


Fig.  110.— Cleavage-planes  cutting  through  strata. 

think  that  we  are  examining  a  beautiful  example  of 
highly  inclined  strata.  But  a  closer  examination  will 
usually  show  the  lines  of  stratification  running  in  an 
entirely  different  direction.     In  Fig.  110,  the  strong  lines 


STRATIFIED  ROCKS.  195 

show  the   strata   strongly  folded,  while  the  light  lines 
show  the  cleavage  nearly  vertical,  cutting  through  these 


Fig.  111.— Strata,  cleavage-planes,  and  joints. 

in  parallel  planes.  In  Fig.  Ill,  three  kinds  of  structure, 
which  should  be  kept  distinct  in  the  mind,  are  shown. 
The  rectangular  block-faces  are  joints  ;  the  strong  lines, 
s  s,  slightly  inclined  to  the  right,  are  strata ;  while  the 
highly  inclined  lighter  lines  are  cleavage-planes  cutting 
through  both. 

Cause  of  Slaty  Cleavage. — Slaty  cleavage  is  undoubt- 
edly caused  by  a  mashing  together  of  the  whole  rock-mass 
in  a  direction  at  right  angles  to  the  cleavage-planes,  and 
an  extension  in  the  direction  of  these  planes  ;  and,  since 
cleavage-planes  are  usually  nearly  vertical,  it  is  the  result 
of  a  mashing  together  horizontally,  and  an  up-swelling  or 
extension  vertically  of  the  whole  cleaved  mass. 

Proof. — This  is  proved  {a)  in  field-observation  by  the 
folding  of  the  strata  (Fig,  110),  and  {h)  in  hand-speci- 
mens by  the  crumpling  of  the  finest  laminae  in  the  direc- 
tion indicated  above.  Fig.  112  represents  a  block  of 
slate  eighteen  inches  long,  in  which  the  lamination-lines 
are  shown  crumpled  by  the  pressure.  In  the  position  of 
the  block  it  is  evident  that  the  crushing  was  horizontal. 
The  pleavage-planes,  represented  by  the  light  lines,  are 
vertical.  One  cleavage-face,  cjo,  is  shown.  The  same  is 
proved,  also  (c),  by  distorted  fossils  often  found  in  cleaved 
slates  (Fig.  113).     By  comparing   the  natural  with  the 


196 


STRUCTURAL  GEOLOGY. 


distorted  form  the  direction  of  pressure  is  found  to  be 
always  at  right  angles  to  the  cleavage-planes,  i.  e.,  the 


Pig.  11S.~A  block  of  cleaved  slate.    (After  Jukes.) 

fossils  are  shortened  in  that  direction  and  elongated  in 
the  direction  of  the  planes  (d).    In  many  slates,  especially 


Fig.  113.— Cardium  hillanum  ;  A,  natural  form  ;  B  and  C,  deformed  by  pressure. 

the  purple  Cumberland  slates,  much  used  in  roofing, 
oblong  greenish  spots  are  common.  If  they  be  closely 
examined,  they  will  be  found  to  be  very  thin  in  the  direo- 


STRATIFIED  ROCKS.  197 

tion  of  the  thickness  of  the  slate  or  at  right  angles  to 
cleavage.     On  the  cleavage  surface  the  shape  is  broad, 
elliptical    (Fig.    114,  A),    while  on   sec- 
tion  the   shape    is    very  flat,    B.     These  ^  B 
spots  before  mashing  were  round  pellets 


/A 


c-£ 


w 


I 


of  clay.     They  have  been  mashed  into  an 

ellipsoid  of  three  unequal  diameters,  the 

longest,  a  h,  in  the  dip  of  the  cleavage, 

and  therefore  nearly  vertical ;  the  next, 

c  d,  in    the   strike  of   the  cleavage,  and 

therefore    horizontal  ;  and  the    smallest,  b  ^ 

e  f,  at   right   angles   to   cleavage.     This    Pig.  ii4.-Fiattened 

,  1      ,      ,  1  IT  1,  1,  nodules  •    A^  face- 

proves  that  the  whole  mass  has  been  view;  5,  side-view, 
mashed  at  right  angles  to  cleavage,  and 
extended  in  the  direction  of  the  dip  of  cleavage.  Micro- 
scopic examination  shows  that  every  constituent  granule 
of  the  original  clay  is  in  the  slate  mashed  into  a  thin  scale, 
so  that  the  original  granular  structure  is  changed  into  a 
scaly  structure,  and  it  is  this  which  determines  the  easy 
splitting. 

Geolog-ical  Application. — The  amount  of  mashing  to- 
gether horizontally  and  extension  vertically  shown  in  these 
different  ways  is  so  great  that  an  original  cube  or  sphere 
in  the  unsqueezed  mass  is  changed  into  an  oblong,  of 
which  the  shortest  diameter  is  to  the  longest  as  one  to 
three  or  four,  one  to  five  or  six,  one  to  nine  or  ten,  and 
even  sometimes  as  one  to  fifteen.  The  average  in  well- 
cleaved  slates  is  one  to  six.  Now,  when  we  remember  that 
thousands  of  square  miles  and  thousands  of  feet  thickness 
of  rocks  are  thus  affected^  it  is  evident  that  this  slow 
mashing  together  horizontally  of  whole  mountain-regions 
must  be  an  important  agent  in  the  elevation  of  land,  and 
especially  in  the  formation  of  mountains.  We  shall  speak 
of  this  again  under  the  head  of  mountains. 


X98 


STRUCTURAL  GEOLOGY. 


Concretionary  or  Nodular  Structure, 

This,  also,  is  a  superinduced  structure  simulating  an 
original  structure.  As  slaty  cleavage  simulates  stratifi- 
cation, so  concretions  or  nodules  simulate  and  are  apt  to 
be  mistaken  for  fossils. 

In  many  strata,  especially  calcareous  sandstones  and 
shales,  we  find  rounded  masses  often  of  curious  shapes, 
separable  from  the  general  mass  of  the  strata,  and  differ- 
ing a  little  from  it  in  hardness  and  composition.  These 
are  called  concretions,  nodules,  septaria,  etc.  They  have 
evidently  been  separated  out  of  the  general  mass  after  the 
latter  was  deposited.     This  is  shown  by  the  fact  that  the 

planes  of  stratification  often 
run  right  through  them 
(Kg.  115). 

Forms  and  Structure. — 
Inform  they  are  sometimes 
perfectly  spherical,  like  can- 
non-balls, and  vary  in  size 
from  that  of  a  marble  to  many  feet  or  even  yards  in  diame- 
ter ;    sometimes   flattened  ellipsoidal,  and   these,  when 


Pig.  115. 


Fig.  lit).— NoUuleb,  from  Ktrata. 


STRATIFIED  ROCKS. 


199 


marked  with  polygonal  cracks,  simulate  very  much  a 
turtle-shell,  and  are  called  turtle-stones  ;  sometimes  dumb- 
bell-shaped, sometimes  rings,  sometimes  all  sorts  of 
strange  and  fantastic  shapes  (Fig.  116).  In  structure 
they  are  sometimes  solid,  sometimes  hollow,  sometimes 
affected  with  interior  cracks,  sometimes  have  a  concentric 
shell-structure,  and  sometimes  a  radiated  structure. 

These  curious  shapes  so  simulate  fossils  that  even  ex- 
perienced geologists  may  sometimes  be  in  doubt.  By 
common  observers  they  are  very  often  mistaken  for  fossil 
nuts,  fossil  turtles,  etc.  They  are,  however,  very  inter- 
esting to  the  geologist,  because  they  often  contain  a  fossil 
beautifully  preserved  in  the  center. 

How  Formed. — They  seem  to  be  formed  by  the  slow 
aggregation  of  more  soluble  or  more  suspensible  matter 
from  a  general  mass  of  insoluble  matter,   an  organism 


\ 


Fig.  117.— Chalk-cliffs  with  flint  nodules. 

often  forming  the  nucleus  of  aggregation.  Thus,  if  the 
mass  be  a  calcareous  sandstone,  the  lime  will  gather  in 
places,  forming  sandstones  containing  more  lime  than  the 
general  mass.  So  calcareous  clays  form  nodules  of  lime 
mixed  witli  clay.  These  are  the  hydraulic-cement  nod- 
ules.    In  chalk  the  disseminated  silica  seems  to  gather 


300  STRUCTURAL  GEOLOGY. 

into  nodules  of  pure  flint,  and  leave  the  chalk  a  pure 
carbonate  of  lime  deprived  of  its  silica.  Hence,  chalk 
usually  contains  flint  nodules,  scattered  or  in  layers  (Fig. 
117). 

We  speak  of  this  nodular  structure  not  on  account  of 
its  great  importance,  but  because  it  is  apt  to  strike  the 
observing  eye,  and  very  apt,  too,  to  be  mistaken  for 
fossils. 

Fossils :  their  Origin  and  Distribution. 

Every  one  must  have  observed  that  in  many  places  the 
stratified  rocks  contain  the  exact  forms  of  organisms, 
especially  shells,  though  these  seem  to  have  turned  to 
stone.  These  are  called  fossils.  They  are  of  extreme 
interest  to  geologists,  because  they  reveal  the  nature  of 
the  former  inhabitants  of  the  earth.  Stratified  rocks  are 
the  consolidated  sediments  of  former  seas,  bays,  lakes, 
and  rivers.  Then,  as  now,  shells  lived  in  the  ooze  of  sea- 
bottoms,  or  were  cast  up  on  beaches ;  the  leaves  and 
branches  of  trees  and  carcasses  of  land-animals  were  car- 
ried down  by  rivers  to  lakes  and  estuaries  and  buried  in 
mud.  These  have  been  preserved,  with  more  or  less 
change,  to  the  present  day.  A  fossil,  then,  may  be 
defined  as  any  evidence  of  the  former  existence  of  a 
living  thing.  Next  to  lamination,  they  are  the  most 
constant  characteristic  of  sedimentary  rocks. 

Degrees  and  Kinds  of  Preservation. — There  are 
various  degrees  and  kinds  of  preservation  of  organic 
forms.  In  some  cases  not  only  form  and  structure,  but 
even  the  organic  matter  of  soft  parts,  is  preserved.  More 
commonly,  however,  only  the  shells  and  skeletons  of  ani- 
mals are  preserved,  and  of  these  sometimes  both  the  form 
and  structure,  and  sometimes  only  the  form.  AVe  shall 
speak  of  these  under  three  heads  : 

1.  Organic  Matter  preserved. — This,  of  course,  is 
rare.      The  only  perfect  examples  are  those  of  carcasses 


I 


STRATIFIED  ROCKS.  301 

preserved  in  ice.  In  the  frozen  clifTs  and  soils  of  Siberia, 
the  carcasses  of  extinct  elephants  and  rhinoceroses  have 
been  exhumed  by  the  rivers,  in  a  condition  so  perfect  that 
dogs  and  wolves  fed  on  the  flesh.  In  peat-bogs  are  found 
the  perfect  skeletons  (still  retaining  the  organic  matter  of 
the  bones)  of  extinct  animals  ;  and  in  some  cases  even  the 
flesh  is  preserved,  but  changed  into  a  fatty  substance 
(adipocere).  These  are  all  in  comparatively  recent  strata. 
But,  even  in  the  oldest  strata,  organic  matters  of  once 
living  beings  are  preserved,  though  changed  into  coal, 
lignite,  petroleum,  bitumen,  etc. 

2.  Organic  Structure  preserved. — This  is  the  type 
of  what  is  called  petrifaction  ;  it  is  best  illustrated  by 
petrified  wood.  In  many  strata,  but  especially  in  the 
sub-lava  gravels  of  California  (page  395)  and  the  tufa 
beds  of  California  and  the  Basin  region,  drift-wood  is 
found  completely  changed  into  stone.  In  these  we  have 
not  only  the  form,  not  only  the  general  structure — i.  e., 
bark,  wood,  and  pith,  concentric  rings,  medullary  rays, 
and  woody  wedges — but  even  the  minutest  microscopic 
structure  of  tissue  and  markings  on  the  walls  of  cells, 
perfectly  preserved  in  the  stony  matter  (usually  silica) 
replacing  the  w^ood. 

Mode  of  Petrifaction. — It  must  not  be  imagined  that 
the  wood  is  turned  to  stone,  but  is  only  replaced  by  stony 
matter.  As  each  particle  of  woody  matter  passes  away 
by  decay,  a  particle  of  mineral  matter  is  deposited  in  its 
place  from  solution,  thus  reproducing  its  structure  per 
fectly.  ,  Wood  best  illustrates  the  process,  but  in  a  simi- 
lar manner  the  minute  structure  of  bones,  teeth,  corals, 
shells,  etc.,  are  preserved,  even  though  the  original  mat- 
ter is  all  gone.  The  most  common  petrifiers  are  silica 
and  carbonate  of  lime. 

3.  Organic  Form  only  preserved. — In  many  cases 
the  structure  is  not  preserved,  but  we  find  only  a  mold  of 
the  external  form,  or  a  cast  of  the  same  in  stone.     This  is 


203 


STRUG  TUBA  L   GEOLOO  Y. 


best  illustrated  by  the  case  of  shells.  The  following 
figure  is  a  diagram  showing  four  different  cases,  all  of 
which  are  very  common.     In  the  figure  the  horizontal 


Fig,  118.— Section  of  strata  containiBg  fossils. 

lines  represent  the  stony  matrix  in  which  the  shell  is 
formed,  or  mud  in  which  the  shell  was  originally  buried, 
and  the  vertical  lines  represent  the  subsequent  filling 
with  finer  material. 

Explanation. — In  case  a,  the  living  or  recently  dead 
shell  was  buried  in  mud,  and  afterward  the  whole  organ- 
ism was  dissolved  and  removed,  leaving  only  the  hollow 
mold  where  it  lay.  In  case  h,  we  have  the  same,  only  the 
mold  has  been  subsequently  filled  and  a  cast  made  by  the 
deposit  of  silica  or  carbonate  of  lime  from  solution.  If 
the  rock  be  broken,  the  cast  will  often  drop  out  of  the 
mold.  In  c,  the  dead,  empty  shell  was  buried  in  mud  and 
filled  with  the  same,  and  afterward  the  shell  was  removed 


Fig.  119.— a,  Natural  form  ;  6, 
cast  of  interior  and  mold  of 
exterior. 


Fig.  ISO.—Trigonia  longa,  showing  cast  (a)  of  the 
exterior  and  (6)  of  the  interior  of  the  shell. 


STRATIFIED  ROCKS.  203 

by  solution,  leaving  an  empty  space  corresponding  to  the 
thickness  of  the  shell.  In  d,  this  hollow  space  was  sub- 
sequently filled  by  deposit  of  soluble  matters  from  perco- 
lating waters.  Cases  c  and  d  are  represented  by  Figs. 
119  and  120. 

Sometimes  we  have  only  the  mold  and  cast  of  a  small 
part  of  an  organism,  as,  for  example,  impressions  of  the 
leaves  of  plants,  or  the  footprints  of  animals  walking  on 
the  mud  when  it  was  soft.  These,  however,  are  of  great 
value,  because  they  are  very  characteristic  parts  of  plants 
and  animals. 

Finally,  there  are  all  grades  of  completeness  of  the 
process  of  replacement.  In  bones,  shells,  and  teeth, 
sometimes  only  the  organic  matter  is  partly  or  wholly 
replaced.  Sometimes,  also,  the  mineral  matter  is  replaced 
by  other  mineral  matter. 

Distribution  of  Fossil  Species. 

The  kind  of  fossils  which  we  find  in  the  strata  at  any 
place  will  depend  on  three  things  :  1.  On  the  hind  of 
rock  ;  2.  On  the  country ;  and,  3.  On  the  age  of  the 
rock. 

Kind  of  Rock. — We  have  already  said  (page  130)  that 
at  the  present  time  different  depths  and  bottoms  are  fre- 
quented by  diiferent  marine  species.  Some  live  on  sand- 
bottoms,  some  on  mud-bottoms,  and  some  on  deep-sea 
ooze.  The  same  was  true  in  previous  epochs,  and  there- 
fore we  ought  to  expect  and  we  do  find  that,  in  the  same 
country,  and  in  strata  of  the  same  age,  sandstones  will 
contain  different  fossils  from  limestones  ;  the  one  being 
shore  and  the  other  open-sea  deposit.  Again,  then  as 
now,  lake-deposits  contained  fresh- water  animals,  and' 
estuary  deposits  land  plants  and  animals  ;  and  these  are 
of  course  different  from  marine  species,  though  they  be  of 
the  same  age  and  country. 

The  Country. — In  rocks  of  the  same  age  and  same 


204  STRUCTURAL   GEOLOGY. 

kind,  but  in  different  continents,  we  shall  often  find  a 
great  difference  of  species,  for  we  find  the  same  thing 
true  of  living  species  (page  118).  But  the  geographical 
diversity  of  fossil  species,  as  a  general  fact,  is  not  so  great 
as  that  of  living  species.  Commencing  with  the  earliest 
times,  the  geographical  differences  of  species  have  in- 
creased more  and  more  to  the  present  time. 

The  Age. — The  distribution  of  fossil  species  according 
to  the  age  of  the  rocks  is  the  main  subject  of  Part  III,  or 
Historical  Geology ;  but  some  general  notions  on  this 
subject  are  necessary  as  a  basis  of  classification  of  strati- 
fied rocks,  and  must  therefore  precede  that  part. 

Successive  Geological  Faunas  and  Floras. — The 
fossil  species  found  in  rocks,  even  of  the  same  kind  and 
country,  will  depend  largely  on  the  age  of  the  rocks. 
The  whole  earth  has  been  inhabited  at  different  times  by 
entirely  different  species.  All  the  animals  and  plants  in- 
habiting the  earth  at  one  time  are  called  the  fauna  and 
flora  of  that  geological  time.  Thus  we  have  a  fauna  and 
flora  of  Tertiary  times,  of  Jurassic  times,  of  Devonian 
times,  etc. 

Definition  of  Formation  and  Period. — When  the 
strata  are  conformable,  the  change  from  one  geological 
fauna  to  another  is  gradual,  but  a  line  of  unconformity 
usually  abruptly  separates  two  faunas.  A  formation, 
therefore,  is  a  series  of  conformable  strata,  in  which  the 
fossil  species  are  either  the  same  or  change  very  gradu- 
ally ;  and  a  geological  period  is  the  period  during  which 
such  a  formation  has  been  laid  down.  There  are  two 
tests,  therefore,  of  the  limits  of  a  geological  formation 
and  a  geological  period,  viz.,  unconformity  of  the  rock- 
system  and  great  change  in  the  species.  Of  these  the 
latter  is  the  more  valuable. 

Law  of  Gradual  Approach  to  the  Present. — It  is  a 
fundamental  and  very  important  fact  that  in  the  suc- 
cessive changes  of   geological  species   there  is  a  steady 


STRATIFIED  ROCKS.  205 

approach  to  living  forms,  first  in  families,  then  in  genera, 
and  then  in  species.  Species  do  not  begin  to  be  identical 
with  the  living  species  until  the  Tertiary  period,  and 
thence  onward  we  have  an  increasing  percentage,  identical 
with  the  living. 

Now,  we  determine  that  rocks  belong  to  the  same  time, 
all  over  the  earth,  by  the  general  similarity  of  the  fossil 
species.  We  find  difficulty  in  applying  this  rule  only  in 
the  Tertiary,  because  then  the  geographical  diversity  is 
beginning  to  be  so  great  as  seriously  to  interfere  with  the 
general  similarity.  But  just  here  we  begin  to  use  another 
principle,  viz.,  the  percentage  of  the  fossil  species  still 
living  in  the  immediate  vicinity.  Similar  percentage  in- 
dicates the  same  age — greater  percentage  less  age,  and 
less  percentage  greater  age.*  It  is  on  these  principles 
that  is  based  the  classification  of  stratified  rocks. 


Section"  II. — Classification-  of  Stratified  Kocks. 

Geology  is  a  history.  Stratified  rocks  are  the  leaves 
of  an  historical  book.  Evidently,  then,  the  true  basis  of 
classification  must  be  relative  age.  In  classification,  the 
geologist  has  two  objects  in  view  :  1.  To  arrange  all  the 
strata,  from  lowest  to  highest,  in  the  order  in  which  they 
were  formed.  2.  Then  to  separate  them  into  groups  and 
sub-groups  for  convenient  treatment — i.  e.,  1.  To  arrange 
the  leaves  in  the  order  in  which  they  were  written,  so  that 
the  story  they  contain  may  be  read  intelligently.  2.  To 
divide  and  subdivide  into  chapters  and  sections,  deter- 
mined by  great  events  in  the  history.  In  a  word,  he  must 
make  first  a  chronology,  and  then  divide  into  eras,  ages, 
periods,  etc. 

Chronology  ;  ^  Order  of  Superposition. — It  is  evi- 
dent, from  the  manner  in  which  sediments  are  formed, 

*  The  teacher  should  consult  the  larger  work,  for  a  complete  state- 
ment. 


206  STRUCTURAL   GEOLOGY. 

that,  if  they  have  not  been  greatly  disturbed,  their  rela- 
tive position  indicates  their  relative  ages,  the  uppermost 
being  of  course  the  youngest.  If,  therefore,  we  have  a 
natural  section  of  strata  (an  exposed  sea-cliif  or  caflon- 
side),  either  horizontal  or  regularly  inclined,  it  is  easy  to 
make  out  the  relative  ages.  But  often  the  rocks  are 
folded  and  crumpled,  and  pushed  over  beyond  the  verti- 
cal ;  they  are  broken  and  slipped,  and  a  large  part  worn 
away  by  erosion  ;  they  are  covered  with  soil  and  hidden 
from  view  ;  so  that  to  make  an  ideal  section  showing  their 
real  relation  is  one  of  the  hardest  of  geological  problems. 
Nevertheless,  if  this  were  all,  we  might  still  hope  for  per- 
fect success.  But  all  the  strata  are  not  represented  in  any 
one  place — usually  only  a  fraction.  Thus,  in  New  York, 
and  all  the  States  westward  as  far  as  the  Plains,  only  the 
older  portion  of  the  record  is  found  ;  while  in  California 
we  have  mostly  the  later  portion.  In  many  places  the 
record  is  still  more  fragmentary.  The  leaves  of  this  book 
are  scattered  about — here,  perhaps,  nearly  a  whole  vol- 
ume ;  there,  one  or  two  chapters  ;  and  yonder,  onlv  a  few 
leaves.  The  geologist  must  gather  these  and  arrange 
them  according  to  their  paging  ;  and  then  divide  anrJ 
subdivide  them  into  volumes,  chapters,  etc.  Therefore, 
although  the  order  of  superposition  must,  wherever  it  can 
be  applied,  take  precedence  of  every  other  method,  yet  it 
must  be  supplemented  by  careful  comparison  of  the  rocks 
in  different  localities  with  one  another.  There  are  tw^ 
means  of  comparison,  viz.,  the  character  of  the  rock  and 
the  character  of  the  fossils. 

Comparison  by  Kock-Character. — This  method  is  of 
little  value  except  in  contiguous  localities.  Sandstones 
of  similar  character  belong  to  nearly  all  times,  and  are 
forming  now.  So,  also,  of  clays  and  limestones.  Coal 
was  once  considered  characteristic  of  a  particular  age,  but 
now  is  known  to  occur  in  strata  of  many  ages.  Chalk 
was  once  supposed  to  be  characteristic  of  the  Cretaceous, 


STRATIFIED  ROCKS.  207 

but  is  now  known  to  be  forming  at  present  in  deep  seas. 
But  since,  both  now  and  in  former  times,  the  same  kind 
of  deposits  formed  over  wide  areas,  rocks  of  similar  kind 
(for  example,  sandstones  of  similar  grain  and  color),  and 
especially  a  group  of  similar  rocks,  in  contiguous  locali- 
ties, are  probably  of  the  same  age.  But  in  widely  sepa- 
rated localities,  as,  for  example,  in  different  continents, 
we  can  not  use  this  method.  To  conclude  that  rocks  are 
of  the  same  age,  because  they  are  of  similar  grain,  color, 
or  composition,  would  almost  certainly  lead  us  astray. 

Comparison  of  Fossils. — This  is  the  most  universal 
and  valuable  means  of  comparison  of  rocks  in  all  parts  of 
the  world.  If  WQJind  a  general  shnilarity  of  species ,  we 
conclude  that  the  rocks  belong  to  the  same  age.  But  we 
must  make  due  allowance — 1.  For  difference  of  conditions 
of  deposit,  whether  shore-deposit  or  deep-sea  deposit, 
whether  fresh-water  or  marine.  2.  We  must  also  make 
due  allowance  for  geographical  diversity.  We  must  ex- 
pect, in  fossils  of  rocks  in  different  continents,  not  abso- 
lute identity,  but  only  general  similarity.  We  shall  find 
little  difficulty  in  applying  this,  until  we  come  to  the 
Tertiary.  But  here  we  have  another  principle  to  help  us, 
viz.,  the  percentage  of  living  invertebrates  found  in  the 
rock.  Vertebrate,  and  especially  mammalian  species,  may 
be  used  in  the  Tertiary  in  much  the  same  way  as  all  species 
in  the  lower  rocks. 

Construction  of  Chronology. — By  application  of 
these  methods,  geologists  in  all  countries,  working  to- 
gether, have  gradually  made  a  nearly  complete  chronol- 
ogy. Breaks  in  one  country  are  filled  by  strata  in  an- 
other. But  a  really  complete  chronology  can  not  be 
expected  until  the  whole  surface  of  the  earth  has  been 
studied,  and  perhaps  not  even  then,  for  some  missing 
links  are  probably  concealed  beneath  the  sea. 

Divisions  and  Subdivisions. — The  next  task  is  to 
divide  and  subdivide  the  whole  into  primary  and  second- 


208 


STRUCTURAL   GEOLOGY. 


ary  groups — into  volumes,  chapters,   etc.,   separated  by 
great  changes.     As  already  explained   (page  204),   there 


Eras. 

Ages. 

Periods, 

Epochs. 

5,  Psychozoic. 

7,  Age  of  Man. 

Human. 

Recent. 

4.  Cenozoic. 

6.  Age  of  Mam- 
mals. 

'  Quaternary. 
Tertiary. 

(  Terrace. 
\  Champlain. 
(  Glacial. 
(  Pliocene. 
\  Miocene. 
(  Eocene. 

8.  Mesozolc. 

Secondary  rocks. 
5,  Age  of  Rep- 
tiles. 

i  Cretaceous. 
•j  J  arassic. 
(  Triassic. 

Upper- 

Carboniferous  "\ 

rocks.          1 

4,  Age  of  Aero-  )■ 

gens  and      | 

Amphibians,  j 

r  Permian. 
J  C'arboniferous. 
1  Siibcarbonifer- 
1^     ous. 

2.  Paleozoic. 

Lower  ^ 

Devonian  rocks. 
3.  Age  of  Fishes. 

'  Catskill. 

Chemung. 
-  Hamilton, 

Corniferous. 
(^Oriskany. 

Silurian  rocks. 
2.  Age  of  Inver- 
tebrates. 

Cambrian  or 
'primordial  rocks. 

'  Helderberg. 
Salina. 
<  Niagara. 
I  Trenton, 
(^Canadian. 
(  Upper. 
\  Middle. 
(  Lower. 

1.  Archaean   or 
ArchaBozoic. 

1.  Archaean  rocks. 

\  Iluronian. 
{  Laurentian. 

STRATIFIED  ROCKS,  209 

aro  two  modes  of  determining  the  limits  of  the  divisions 
of  the  rocks,  and  correspcnanig  divisions  of  time,  viz., 
by  unconformity  of  the  rocks,  and  by  change  of  the  fossils. 
These  two  usually  occur  toge^lier,  because  they  are  pro- 
duced by  ^Iie  same  cause,  viz.,  change  in  physical  geogra- 
phy and  climate ;  but,  if  there  be  discordance  between 
the  two,  then  we  follow  the  change  in  the  fossils  rather 
than  unconformity  of  rocks.  By  means  of  the  most  gen- 
eral unconformity  and  greatest  change  in  fossil  forms,  the 
primary  divisions  are  established  ;  and  then,  by  less  gen- 
eral unconformity  and  less  important  changes  in  organic 
forms,  these  are  divided  and  subdivided.  A  generalized 
schedule  of  the  divisions  and  subdivisions  of  the  rocks 
and  corresponding  divisions  of  time  which  will  be  used 
in  this  work,  is  given  on  the  preceding  page. 

Lb  Cokte,  Qbol.  14 


CHAPTER  III. 

UNSTRATIFIEI?    OR    IGN^EOUS   ROCKS. 

These  differ  wholly  from  the  stratified  rocks — 1  By 
absence  of  true  stratification,  i.  e.,  lamination  by  sorting 
of  material.  2.  By  absence  of  fossils.  3.  By  a  crystal- 
line or  else  a  glassy  texture  instead  of  an  earthy  texture. 
4,  By  mode  of  occurrence,  as  explained  below. 

Origin. — All  these  characteristics  are  the  result  of  their 
mode  of  origin.  They  have  consolidated  from  a  state  of 
fusion  or  semi-fusion,  and  poured  out  from  helow,  instead 
of  deposited  as  sediments  from  above.  Their  original 
fused  condition  is  shown  by  their  crystalline  or  glassy 
texture,  by  their  occurrence  injected  into  fissures,  or  even 
tortuous  cracks,  and  by  their  effects  on  the  stratified  rocks 
with  which  they  come  in  contact. 

Mode  of  Occurrence. — They  occur  in  three  main 
positions  :  1.  Underlying  the  stratified  rocks  and  appear- 
ing on  the  surface  in  great  masses,  especially  in  mountain- 


'0^-'^^-  ,"-  ^  ,^       |iiiiii|  Eruptives. 
^^^^'C'  \  t  r!X  '-^        •»  Wi^  Granitics.  • 

!<;i:>^i  ^  /  ,  s^  / .  ^  ,  '  l:^::::!  Metamorphic. 


Palaeozoic. 
p^  Mesozoic. 
Cenozoic. 


Fig.  121.— Ideal  section  of  the  earth's  crust. 


i 


UNSTRATIFIED   OR  IGNEOUS  ROCKS.  211 

ous  regions  {a,  Fig.  121).  2.  In  vertical  sheets  intersect- 
ing the  stratified  rocks  or  other  igneous  rocks,  b.  3.  In 
streams  or  sheets  overlying  the  stratified,  or  else  between 
the  strata,  c  c\  4.  Sometimes  as  tortuous  veins,  d  d, 
connected  with  the  great  underlying  masses.  All  of  these 
are  connected  with,  and  are  extensions  of,  the  great 
underlying  masses. 

Extent. — iVs  thus  defined,  igneous  rocks  occupy  hut  a 
small  portion,  certainly  not  more  than  one  tenth,  of  the 
land-surface.  But  beneath  the  stratified  rocks  they  are 
supposed  to  form  the  great  mass  of  the  earth. 

Classification  of  Igneous  Rocks. — Igneous  rocks  can 
not  be  classified,  like  sedimentaries,  by  relative  age. 
They  are  best  classified  partly  by  texture  and  partly  by 
mode  of  occurrence.  They  thus  fall  into  two  strongly 
contrasted  groups,  viz.,  plutonics  and  volcanics,  or  gra- 
nitics  and  true  eruptives.  The  rocks  of  the  one  group 
are  very  coarse-grained  and  wholly  crystalline,  of  the 
other,  finer-grained  or  even  glassy.  The  one  occurs  only 
in  great  masses,  either  underlying  the  stratified  rocks,  or 
appearing  on  the  surface  over  wide  areas,  especially  in  the 
axes  of  mountain-ranges  ;  the  other,  in  sheets  injected 
among  the  strata,  or  as  streams  and  sheets  outpoured  on 
the  surface.  The  granitics  have  not  usually  been  erupted 
at  all,  although  they  often  form  the  reservoirs  from  which 
eruptions  have  taken  place. 

It  is  sometimes  convenient  to  speak  of  an  intermediate 
group — trappean.  If  so,  then  the  three  kinds  correspond 
to  the  three  positions  mentioned  above.  The  granitic 
(Fig.  121,  a)  occur  beneath ;  the  trappean,  bb,  injected 
among ;  the  volcanic,  cc,  outpoured  upon,  the  stratified 
rocks. 

I. — The  Massive  or  Granitic  Group. 

The  rocks  of  this  group  occur  in  great  masses,  not  in 
sheets  or  streams.     They  are  all  very  coarse-grained  in 


212  STRUCTURAL   GEOLOGY. 

texture,  and  have  a  speckled  or  mottled  appearance,  be- 
cause composed  of  crystals  of  considerable  size,  and  of 
different  colors,  aggregated  together.  The  crystals  of 
which  they  mainly  consist  are,  quartz,  feldspar,  mica, 
and  hornblende.  In  such  a  coarse,  speckled  rock,  the 
bluish,  glassy,  transparent  spots  are  quartz  ;  the  opaque, 
whitish,  or  rose  or  greenish  crystals,  with  striated  surface, 
are  feldspar ;  the  black  spots  are  usually  hornblende  ;  the 
mica  may  be  known  by  its  thin,  scaly  structure,  some- 
times pearly,  sometimes  black. 

The  whole  group  is  called  granitic,  because  granite  is 
its  best  type.     In  popular  language,  indeed,  all  these  rocks 

would  be  called  granite,  but  sci- 
ence makes  a  difference.  If  the 
rock  consists  of  quartz,  feld- 
spar, and  mica,  or  else  of  these 
with  hornblende,  then  it  is 
granite  proper.  If  it  consists 
of  feldspar  and  hornblende,  or 
Fig.  ^2.-Xaphic  gTI^te.  ^^^ese  with  quartz,  it  is  called 
syenite.  If  it  consists  of  only 
quartz  and  feldspar,  and  the  quartz  be  m  bent  plates, 
looking,  on  section,  like  Hebrew  characters,  it  is  called 
pegmatite  (Fig.  122).  The  feldspar  in  all  these  is  potash- 
feldspar,  or  ortlioclase.  Diorite  is  a  dark,  speckled  rock 
of  the  same  composition  as  syenite,  except  that  the  feld- 
spar is  a  soda-lime  feldspar  or  plagioclase.  Gahhro  and 
diabase  are  dark -greenish  rocks  similar  to  diorite,  except 
that  the  hornblende  is  replaced  by  augite  and  olivine."^ 

Mode  of  Occurrence. — The  mode  of  occurrence  of 
these  rocks  has  been  already  explained.  They  never 
occur  in  overflows.  They  rarely  or  never  occur  in  in- 
truded sheets  or  dikes.  They  occur  only  in  great  masses. 
or  sometimes  in  tortuous  veins  closely  connected  with  the 

*  The  teacher  must  have  a  small  collection  of  rocks  and  of  min- 
erals for  illustration. 


UNSTRATIFIED  OR  IGNEOUS  ROCKS.  213 

great  masses,  as  if  forced  into  cracks  by  heavy  pressure 
(Fig.  121,  d).  Their  coarsely  crystalline  texture  and 
their  mode  of  occurrence  are  well  explained  by  supposing 
that  they  have  cooled  at  great  depth  in  lai-ge  masses,  and 
consequently  slowly.  When  they  appear  at  the  surface, 
therefore,  they  have  been  exposed  by  extensive  erosion. 

Two  Sub-Grovips. — All  igneous  rocks,  whether  plu- 
tonic  or  volcanic,  are  divisible  into  two  sub-groups,  acidic 
and  basic.  In  the  acidic,  quartz  and  potash-feldspar 
(orthoclase)  predominate  ;  in  the  blasic,  hornblende  or 
augite  and  soda-lime  feldspar  (plagioclase)  predominate. 
The  rocks  of  the  former  group  are  lighter  colored  and 
less  dense  ;  of  the  latter,  are  darker  and  heavier ;  but  the 
two  sub-groups  run  insensibly  into  each  other.  Among 
the  granitics,  granite  is  the  best  type  of  the  acidics  ;  and 
diorite,  and  especially  gabbro  or  diabase,  of  the  basics. 

Intermediate  Series. 

Between  the  true  plutonics  and  true  volcanics  there  is 
an  intermediate  series,  called  trappean  or  intrusives.  If 
the  plutonics  occur  in  masses  beneath,  the  volcanics  in 
outpoured  streams  and  sheets  upon,  these  occur  in  sheets 
intruded  among,  the  strata, 
especially  of  the  older  rocks. 
They  are  finer-grained  than 
the  plutonics  and  more  crys- 
talline than  volcanics.  The 
reason,  apparently,  is  that  they 
have  cooled  more  rapidly  than 
the  former,  and  less  rapidly 
than  the  latter.  These  are 
also  divisible  into  acidics  and 
basics.      Amonff    the    acidics     ^^^-  ^^-^  p'^^^  ""^  porphyry 

1,  .  ,    .^  J  (after  Lyell). 

would   come  felsite  and  por- 
phyry, and,  among  basics,  diorite  and  diabase,  for  these 
occur  both  massive  and  intrusive. 


214 


STRUCTURAL  QEOLOQY. 


Diorite  and  diabase  have  already  been  described.  It  is 
only  necessary  to  say  that,  when  occurring  intrusive, 
they  are  finer-grained  than  the  massive  varieties.  Felsite 
is  a  fine-grained,  light-grayish  rock,  consisting  essentially 
of  orthoclase  and  quartz.  Porphyry  is  a  rock  consisting 
of  fine-grained  feldspathic  paste,  with  disseminated  large 
crystals  of  feldspar  (Fig.  123).  But  any  rock  is  said  to  be 
porphyritic  if  it  consists  of  fine-grained  paste  with  large 
crystals  of  any  kind  disseminated. 

II. — VoLCAifics,  OR  True  Eruptives. 

The  rocks  of  this  group  are  distinguished  from  those 
of  the  other,  both  by  texture  and  mode  of  occurrence.  By 
texture  they  are  not  only  finer-grained  (micro-crystal- 
line), but  there  is  always  more  or  less  of  uncrystalline  or 
glassy  base  or  cement,  showing  that  the  fused  mass  has 
cooled  too  quickly  to  allow  complete  crystallization.  Often, 
also,   as   already   explained  under  volcanoes  (page  139), 

these  rocks  are  in 
a  wholly  glassy  and 
even  in  a  scoriaceous 
and  tuf aceous  condi- 
tion. The  principal 
rocks  of  this  group 
are  given  in  the  ac- 
companying table. 

Trachyte  may  be 
taken  as  a  type  of 
the  acidics.  It  is  a  light-colored  rock,  with  a  rough  feel 
(hence  the  name),  consisting  essentially  of  orthoclase  with 
more  or  less  quartz.  When  the  quartz-grains  are  con- 
spicuous, it  becomes  rhyolite.  Phonolite  is  a  dense  vari- 
ety, of  light-grayish  color,  which  splits  into  slabs  in 
weathering,  and  rings  under  the  hammer  almost  like 
metal  (hence  the  name).  Obsidian  and  pumice  are  glassy 
and  scoriaceous  varieties  of  trachyte. 


VOLCANIC   ROCKS. 

ACIDIC. 

BASIC. 

i  Rhyolite. 
Stony.  }  Trachyte. 
(  Phonolite. 

Glassy.  \  Obsidian. 
(  Pumice. 

Basalt. 

Dolerite. 

Andesite. 

Tachylite. 
Black  scorisB. 

UNSTRATIFIED   OR  IGNEOLS  ROCKS. 


215 


Basalt  is  the  type  of  the  basics.  It  is  a  very  dark, 
almost  black,  heavy  rock,  scarcely  visibly  grained  to  the 
naked  eye,  and  breaking  with  conchoidal  fracture.  It 
consists  of  plagioclase  with  augite,  olivine,  and  magnetite. 
Dolerite  has  a  similar  composition,  but  more  distinctly 
crystalline  texture,  and  therefore  dark-grayish  color. 
Tachylite  is  the  glassy  variety,  which,  if  vesicular,  be- 
comes black  scoria. 

The  following  table  is  a  condensed  statement  of  the 
composition  of  the  principal  kinds  of  rocks  numbered 
above.     The  sign  x  x  indicates  crystals. 

IGNEOUS  ROCKS. 


-   u^ 

1P 


Is 


Rhyolite. 
Vitreous 


+ 

X   X  of 
Quartz, 
Orthoclase 

(sanidin). 


Trachyte. 
Vitreous 


X    X  of 
Orthoclase 
(sanidin). 


Phonolite. 

Vitreous 

base. 

+ 

X  X  of 
Sanidin, 
Nephelin. 


Quartz-porphyry. 
Micro  X    X  ground- 
mass. 

+ 
X    X  of 
Orthoclase, 
Quartz. 


Fdmte. 


Micro  X   X  of 
Orthoclase, 
Quartz. 


Andesite. 

Vitreous 

base. 

+ 
X   X  of 
Plagioclase, 
Augite,  or 
Hornblende. 


Basalt. 

Vitreous 

base. 

+ 

X      X  of 

Plagioclase, 

Au^te, 

Olivine. 


Biorite. 
See  below. 


Diabase. 
See  below. 


3  2 


Granite. 
X   X  of 
Quartz, 
Orthoclase, 
Mica. 


Syenite. 

X  X  of 
Orthoclase, 
Hornblende. 


Diorite. 
X    X  of 
Plagioclase, 
Hornblende. 


Diabase. 
X   X  of 
Plagioclase. 
Augite. 


Two  Modes  of  Eruption. — There  are  two  modes  of 
eruption.  In  the  one,  the  fused  mass  comes  up  through 
chimneys,  and  flows  off  in  streams  (or  ejected  as  cinders 
and  ashes)  ;  in  the  other,  it  comes  up  through  great 
fissures   often  hundreds   of  miles  long,  and  spreads  as 


216"  STRUCTURAL  GEOLOGY. 

extensive  sheets.  In  the  one  the  erupted  matters  ac- 
cumulate about  the  vent  as  a  cone  ;  in  the  other  they 
form  great  lava-fields,  or  else  may  be  forced  between  the 
strata  and  never  come  to  the  surface  at  all.  In  the  one 
ih.Q  force  of  ejection  is  probably  the  elastic  force  of  vapors, 
as  explained  under  volcanoes ;  in  the  other  the  force  is 
more  obscure,  but  probably  of  the  same  nature  as  that 
y\h\Gh.  forms  mountains.  The  two  kinds  may  be  called 
crater -eruptions  and  fissure-eruptions.  At  present  only 
the  former  kind  seems  to  exist ;  and  therefore  in  Part  I, 
while  treating  of  causes  now  in  operation,  we  treated  only 
of  this  mode.  But  in  studying  erupted  materials  of  all 
periods,  it  is  plain  that  by  far  the  larger  quantity  have 
come  up  in  the  second  way. 

Modes  of  Occurrence. — Leaving  out  of  view  those 
modes  of  occurrence  already  described  under  volcanoes, 
viz.,  chimney-cones  with  radiating  dikes  and  lava-streams, 
the  principal  modes  of  occurrence  of  eruptive  rocks  are  : 
1.  Dikes.     2.   Overflow-sheets.     3.  Intercalary  teds. 

1.  Dikes. — Dikes  are  vertical  sheets  filling  great  fis- 
sures in  stratified  or  other  igneous  rocks.  They  are  the 
most  common  of  all  modes  of  occurrence  of  eruptives  and 
intrusives.  In  all  mountain-regions  they  are  found  in 
great  numbers.  In  width  they  vary  from  a  few  feet  to 
hundreds  of  feet,  and  may  often  be  traced  outcropping 
over  the  surface  fifty  to  one  hundred  miles.  But  since 
rocks  are  usually  covered  with  soil,  they  are  not  always 
visible  at  once,  but  must  be  looked  for  wherever  the  rock 
is  exposed,  especially  in  stream-beds. 

It  is  evident  that  fused  matter  coming  to  the  surface 
must  overflow,  and  therefore  dikes  thus  outcropping  on 
the  surface  are  either  the  exposed  roots  of  former  over- 
flows which  have  been  removed  by  erosion,  or  else  are  the 
fillings  of  fissures  which  never  reached  the  surface  at  all 
(Fig.  121,  V).  In  either  case,  an  outcropping  dike  is  the 
sign  of  gieat  erosion.     If,  therefore,  the  dike  is  harde:r 


'^:^:rt^--S-  - 

i   A 

rrCTJ-I^t^^ 

jL=i 

^^"_="-=-r 

f^^^^ 

p^TrTj 

i^m^ 

IUNSTRATIFIED   OR  IGNEOUS  ROCKS.  217 

han  the  country-rock  through  which  it  breaks,  it  will 
tand  above  the  surface  and  look  like  a  low,  ruined  wall 

Fig.  124.— Dikes. 

(Fig.  124,  a).  If,  on  the  contrary,  the  igneous  rock  yield 
more  easily  to  erosion  than  the  country-rock,  then  it  may 
be  traced  as  a  shallow,  half -filled  ditch  (Fig.  124,  b). 
^:  EflPect  of  Dikes  on  Stratified  Rocks. — On  both 
sides  of  a  dike  the  bounding  walls  of  stratified  rock  are 
always  changed  by  the  intense  heat  of  the  fused  matter. 
Sandstones  are  changed  into  a  rock  resembling  gneiss 
(page  225),  clays  are  baked  into  porcelain  jaspers,  lime- 
stones are  changed  into  crystalline  marbles,  coal-seams 
into  anthracite  and  sometimes  into  coke.  In  all  cases  the 
fossils,  if  any,  are  more  or  less  completely  destroyed. 
These  metamorphic  changes  usually  extend  only  a  few 
feet  or  yards  from  the  place  of  contact. 

2.  Overflows. — This  is  the  next  most  common  form  of 
occurrence.     The  liquid  matter  has  come  up  through  great 


riiiiiiiiiiiiiuuuiiiiiiiiiiii 


Fig.  125.— Lava  sheets. 


fissures,  such  as  are  made  by  crust-movements,  and  spread 
on  the  surface  as  extensive  sheets.     Often  sheet  after  sheet 


218 


STRUCTURAL   GEOLOGY. 


is  outpoured,  one  on  another,  until  masses  2,000  to  3,000 
feet  thick  are  piled  up  (Fig.  125). 

The  extent  and  thickness  of  some  of  these  lava-floods 
are  almost  incredible.  The  great  lava-flood  of  the  North- 
west covers  the  whole  of  northern  California,  north- 
western Nevada,  and  a  great  part  of  Oregon,  Washington, 
and  Idaho,  and  extends  far  into  Montana  and  British 
Columbia.  Its  area  is  supposed  to  be  150,000  square 
miles,  and  its  thickness,  where  cut  through  by  the  Co- 
lumbia  River,  is  at  least  3,000  feet.  There  are  about  a 
dozen  extinct  volcanoes  dotted,  at  wide  intervals,  over 
this  vast  area.  It  seems  certain  that  the  lava  came  up 
through  fissures  in  the  Cascade  and  Blue  Mountains,  and 
spread  as  sheets  which  covered  the  whole  intervening 
space.  Afterward  eruptive  activity  continued,  in  a  more 
feeble  form  as  volcanoes,  almost  to  the  present  time. 
The  great  Deccan  lava-field,  described  by  the  geologists 
of  India;  covers  an  area  of  200,000  square  miles,  and  is  in 
places  6,000  feet  thick,  and  there  is  no  evidence  of  any 
crater-eruptions  at  all. 

These  very  extensive  sheets  are  usually  basalt.  In 
some  parts  of  the  Utah  and  Nevada  Basin  region,  how- 
ever, rhyolitic  and  trachytic  lavas  are  found  7,000  feet 
thick,  but  these  are  far  less  extensive.  As  a  general  rule, 
the  basic  lavas,  like  basalt,  were  very  liquid  (superfused), 
and  spread  out  in  thin  sheets,  while  the  acidic  lavas,  like 
trachyte,  have  been  stiffly  viscous  (semi-fused),  and  were 
squeezed  out  dome-shaped. 


■     .iTmmngaUU^ 

Hjj  IlillMITTTTrmTTrrrf^  —^ 

^zrhjUllllllliiiimiJp^ — J 

^:^«x.x,:^.uxujiiiii^y^^^^ 

— oTTTTniiiiiiiiiiiiirt — mjjm^^*^-- 

Fig.  126.— lutercalary  beds. 


UNSTRATTFTED   OR  IGNEOUS  ROCKS. 


219 


3.-  Intercalary  Beds. — Often  sheets  are  found  be- 
tween the  strata,  sometimes  repeated  many  times.  In 
such  cases  they  may  have  been  poured  out  on  the  bed  of 
the  sea  or  lake,  and  covered  with  sediment ;  or  they  may 
have  broken  through  the  strata  for  a  certain  distance, 
and  then  spread  between  tbe  separated  strata  (Fig.  126). 
Both  of  these  cases  occur.  If  the  strata  both  above  and 
below  the  sheet  are  changed  by  heat,  then  it  has  been 
forced  between  ;  but  if  only  the  underlying  stratum  is 
changed,  then  it  has  been  outpoured  on  the  bed  of  the 
'sea  or  lake,  and  covered  with  sediment. 

Age  of  Erupt ives. — Where  two  dikes  or  streams 
meet,  their  relative  ages  may  be  known.  In  case  of  suc- 
cessive streams,  that  which  covers  is  of  course  the  later. 
If  one  dike  intersects  another  (Fig.  127),  the  intersecting 
dike,  «,  is  the  younger.     The  absolute  age,  i.  e.,  the  geo- 


FiG.  127. 


logical  period  when  the  eruption  took  place,  can  be  de- 
termined only  by  the  age  of  the  associated  stratified  rocks. 
If  igneous  rocks  break  through,  or  are  outpoured  upon, 


Fig.  128. 


or  forced   between  layers    of   stratified    rocks,   then  the 
igneous  rock  must  be  younger  ;  but  if  intercalary  beds 


230 


STRUCTURAL   GEOLOGY. 


are  the  result  of  outpouring  on  the  bed  of  the  sea,  and 
covering  it  with  sediment,  then  the  igneous  and  the 
stratified  rocks  are  contem'poraneous.  Finally,  if  dikes 
outcropping  on  the  surface  are  covered  with  other  strata 
through  which  they  do  not  break  (Fig.  128),  then  they 
are  younger  than  the  lower  series,  a,  and  older  than  the 
upper,  5. 

Some  Structures  common  to  Many  Eruptives, 

Columnar   Structure. — Many   eruptive   rocks,   espe= 
cially  of  the  more  basic  kinds,  seem  to  be  wholly  made 


Fig.  liJ9.— Columnar  basalt,  New  South  Wales  (Dana). 

up  of  regular  prismatic  columns  (Fig.  129).  This  re- 
markable structure  is  most  common  and  perfect  in  basalt, 
and  is  therefore  often  called  basaltic  structure.  The  col- 
umns vary  in  size  from  a  few  inches  to  several  feet  in 
diameter,  and  in  length  from  a  few  feet  to  one  hundred 
feet ;  the  number  of  sides  from  three  to  seven,  more  com- 


I'lu.  130.— Basaiuc  columns  (after  Geikie). 


UNSTRATIFIED  OR   IGNEOUS  ROCKS. 


221 


monly  five  or  six-  The  columns  are  not  usually  continu= 
ous,  but  short- jointed,  like  a  vertebral  column  (Fig,  130), 
T\iQ  position  of  the  columns  is  usually  perpendicular  to 
the  cooling  surface.  Thus,  in  vertical  sheets,  like  dikes, 
they  are  horizontal,  and  an  outcropping  dike  often  pre- 
sents the  appearance  of  a  pile  of  corded  wood  (Fig.  131), 


FiG.  131.— Columnar  dike.  Lake  Superior.    (After  Owen.) 


In  overflow-sheets  the  columns  are  vertical  (Fig,  129), 
and  at  the  base  of  a  cliff  of  such  rocks  are  found  piles  of 
separated  and  disjointed  columns. 

The  cause  of  this  structure  is  shrinkage  by  cooling. 
Many  substances  shrink  by  drying,  and  break  into  pris= 
matic  columns.  Mud  thus  forms  polygonal  prisms  by 
sun-cracks.  Wet  starch,  poured  into  boxes  and  drying, 
breaks  into  prismatic  pencils.  In  the  case  of  lava,  the 
shrinkage  is  by  cooling,  instead  of  drying,  and  the  prisms 

Wt    ftre  far  more  regular, 

H|      Examples  of  this  structure  are  found  in  every  country. 


222 


STRUCTURAL   GEOLOGY. 


Giant^s  Causeway  in  Ireland,  and  FingaFs  Cave  on  the 
Island  of  Stalia,  are  good  examples.  The  Giant's  Cause- 
way is  a  feea-cliff  of  columnar  basalt,  consisting  of  many 
layers,  with  softer  material  between,  and  the  whole  rest- 
ing on  stratified  rock.  By  the  action  of  the  sea  and  air 
the  separated  and  disjointed  columns  are  undermined  and 
fall  to  the  base  of  the  cliff.  In  this  country,  the  Pali- 
sades of  the  Hudson  River,  and  Mounts  Tom  and  Hol- 
yoke  in  the  Connecticut  River  Valley,  are  good  examples. 
Fine  examples  are  found  also  in  the  trap  of  Lake  Supe- 
rior (Fig.  132).     But  the  finest  in  this  country  are  the 


Fig.  132.— Basaltic  columns  on  sedimentary  rock,  Lake  Superior.     (After  Owen.) 

basaltic  cliffs  of  Columbia  and  Des  Chutes  Rivers  in  Ore- 
gon. On  the  Des  Chutes  River  at  least  thirty  lava-layers 
may  be  counted,  one  above  another,  each  entirely  com- 
posed of  vertical  columns. 

Volcanic  Cong-lomerate  and  Breccia. — If  a  lava- 
stream  runs  down  a  stream-bed  or  a  shingly  beach,  it 
gathers  up  the  pebbles  and  forms  with  them  a  conglome- 
rate differing  from  aqueous  conglomerate  in  the  fact  that 
the  uniting  paste  is  igneous  instead  of  sedimentary.  So, 
also,  a  lava-stream  may  gather  up  rubble  and  form  a 
volcanic  breccia  differing  in  the  same  way  from  sedimen- 
tary breccia. 


b 


UNSTRATIFIED   OR  IGNEOUS  ROCKS.  223 

Amygdaloid. — The  upper  part  of  a  lava-stream  is  ve- 
sicular, or  full  of  air-bubbles.  If  such  a  stream  be  cov- 
ered by  another  stream,  percolating  waters,  charged  with 
silica  and  carbonate  of  lime  gathered  from  the  lava,  will 
fill  up  the  empty  spaces  with  these  materials.  If  the  rock 
be  broken  or  weathered,  these  amygdules  fall  out.  They 
hoh  somewhat  like  pehhles,  and  the  rock  (Fig.  133)  might 
be  mistaken  for  conglomerate,  but  is  formed  in  an  entirely 


Fir,.  133.— Amygdaloid. 

different  way.     The  filling  of  the  cavities  takes  place 

slowly,  layer  within  layer,  and  the  layers  are  often  of  dif- 
ferent colors.  It  is  in  this  way  that  are  formed  the  most 
exquisite  agate  and  carnelian  nodules. 

Tufas. — When  volcanic  materials  disintegrate,  and  are 
then  moved  and  deposited  in  water,  they  form  tufas. 
Sometimes  the  fragments  may  be  larger  and  the  mass  may 
simulate  volcanic  breccia.  It  is,  however,  an  aqueous 
breccia  of  volcanic  rock.  Such  are  sometimes  called  vol- 
canic agglomerates. 


CHAPTER   IV. 

METAMORPHIG    ROCKS. 

"We  have  now  finished  both  the  stratified  and  the  un- 
stratified  rocks,  but  there  is  yet  an  intermediate  series 
which  must  be  described.  These  are  stratified  like  the 
stratified  rocks,  but  crystalline  in  texture,  and  usually 
destitute  of  fossils,  like  the  igneous  rocks.  They  are 
supposed  to  have  been  formed  from  sediments  like  strati- 
fied rocks,  but  have  been  subsequently  changed  by  heat 
and  other  agencies.  They  are  therefore  called  meta- 
morpJiic  rocks.  They  may  be  traced  by  gradations,  on 
the  one  hand,  into  stratified,  and,  on  the  other,  into 
igneous  rocks. 

Extent  and  Thickness. — They  cover  large  areas,  es- 
pecially among  the  oldest  rocks  and  along  axes  of  great 
mountain-chains.  The  whole  of  Labrador,  the  larger  por- 
tion of  Canada,  the  whole  eastern  slope  of  the  Appala- 
chian, and  also  the  axes  of  the  Colorado  and  Sierra,  con- 
sist of  them.  In  Canada  they  are  supposed  to  be  40,000 
to  50,000  feet  thick  and  very  much  crumpled.  Meta- 
morphism  is  nearly  always  associated  with  great  thickness 
and  crumpling. 

Age. — The  oldest  rocks  are  all  metamorphic.  Hence 
many  regard  it  as  a  sign  of  age.  But  it  is  probably  more 
correct  to  say  that  metamorphism  is  found  in  rocks  of  all 
ages  if  only  they  be  very  thick  and  very  much  crumpled  : 
but,  since  great  thickness  and  complex  crumplings  are 
most  common  in  the  oldest  rocks,  so  also  is  metamorphism. 
224 


METAMORPHIC  ROCKS,  225 

Kinds.— The  adjoining  table  shows  the  principal  kinds: 
Gneiss  is  a  rock  having  much  the 
appearance  and  mineral  composition 
of  granite — i.  e.,  quartz,  feldspar, 
mica,  and  hornblende — differing 
only  in  a  bedded  structure.  In  many 
places,  as,  for  example,  on  Manhattan 
Island,  gneiss  can  be  traced  by  in- 
sensible gradations  into  granite. 
Schists  are  rocks  having  a  fissile 
structure    through     the    abundant 


Gneiss. 

Mica-schist. 

Chlorite-schist. 

Talcose-scliist. 

Hornblende-schist. 

Clay-slate. 

Quartzite. 

Marble. 

Serpentine. 


i 


presence  of  scales  of  some  kind.  In  mica-schist  they 
are  mica ;  in  the  other  schists  they  are  chlorite,  or  talc, 
or  hornblende. 

Quartzite  and  Marble  are  both  white,  crystalline,  or 
granular  rocks,  looking  like  loaf-sugar ;  but  in  the  one 
case  the  granules  are  quartz,  in  the  other,  lime-carbonate. 
SeriwMtine  is  a  greenish  rock,  having  usually  a  schistose 
structure  and  a  greasy  feel  like  talc.  It  contains  a  nota- 
ble quantity  of  magnesia. 

Origin  of  these  Kinds. — Metamorphic  rocks  are  prob- 
ably changed  sandstones,  limestones,  and  clays,  and 
mixtures  of  these.  The  infinite  variety  which  we  find  is 
the  result  partly  of  the  original  kind  and  partly  of  the 
degree  of  change.  For  example,  sandstones  and  lime- 
stones are  often  perfectly  pure.  Now,  a  metamorphic 
pure  sandstone  is  quartzite,  and  a  metamorphic  pure  lime- 
stone is  marble.  But  clays  are  nearly  always  impure, 
being  mixed  with  sand  and  lime  and  iron  and  other  bases. 
A  moderately  pure  clay  with  a  little  sand  by  metamor- 
phosis makes  gneiss  or  mica-schist.  If  it  contains  much 
iron,  it  makes  a  hornblende-schist ;  if  magnesia,  talcose- 
schist  or  serpentine.  Serpentine  is,  however,  often  a 
changed  eruptive  rock. 

Le  Oonte,  Geol.  15 


22G  STRUCTURAL   GKOLOOY. 

Cause  of  Metamorphism, 

There  are  two  kinds  of  metamorphism  which  must  be 
distinguished,  viz.,  local  or  contact  metamorphism,  and  re- 
gional metamorphism.  The  former  is  produced  by  direct 
contact  with  fused  matter,  as  in  dikes  or  intercalary  beds 
(page  217).  There  can  be  no  doubt  as  to  the  cause  in 
this  case.  It  is  intense  heat.  But  the  effect  of  the  heat 
extends  but  a  little  way  from  the  plane  of  contact.  In 
regional  metamorphism,  on  the  contrary,  the  change  is 
universal  over  hundreds  of  thousands  of  square  miles  and 
thousands  of  feet  of  thickness.  In  these  cases  there  is  no 
evidence  of  intense  heat  in  every  part ;  the  heat  was  prob- 
ably very  moderate.  It  is  of  this  kind  that  we  now  wish 
to  explain  the  cause. 

The  Ag-ents  of  regional  metamorphism  are — 1.  Heat ; 
2.  AVater ;  3.  Alkali ;  4.  Pressure  ;  5.   Crushing. 

To  produce  metamorphism  by  lieat  alone,  i.  e.,  dry  heat, 
would  require  a  temperature  of  2,500°  to  3,000°,  but  in  the 
presence  of  waters  very  moderate  heat  will  change  rocks. 
At  400°  Fahr.  (=  205°  C),  incipient  change  commences  ; 
and  at  800°  Fahr.,  complete  hydrothermal  fusion  takes 
place.  If  any  alkaline  carbonate  be  present  ii?  the  water, 
these  effects  occur  at  still  lower  temperature.  The  quan- 
tity of  water  necessary  is  only  ten  to  fifteen  per  cent.  ;  in 
other  words,  the  included  loater  of  sediments  is  amply 
sufficient.  Pressure  is  necessary,  because  it  is  impossible 
to  have  even  such  moderate  heat  in  the  presence  of  water, 
unless  the  whole  be  under  pressure. 

Application. — Suppose,  then,  we  have  sediments  ac- 
cumulating along  a  shore-line,  or  at  the  mouth  of  a  river 
until  a  thickness  of  10,000,  20,000,  or  40,000  feet  is 
reached.  It  is  evident  that  the  isogeotherms  (interior 
isotherms)  would  rise,  and  the  lower  portion  of  the  sedi- 
ments with  their  included  waters  would  be  invaded  by 
the  interior  heat  of  the  earth  (Fig.  134).     At  the  rate  of 


MMTAMOBPHIG  ROCKS, 


227 


100°  increase  per  mile  (page  132),  the  lower  portion  of 
the  sediments  20,000  feet  thick  would  be  400°  +  60° 
(mean  surface  temperature)  =  400°,  and  40,000  feet  of  the 
sediments  would  be  at  the  bottom  860°.     Now,  we  actually 


Fio.  134. — «6,  original  sea-bottom  ;  s'b',  sea-bottom  after  sediments,  sd,  have  accu- 
mulated ; ,  isogeotherms  of  800°  and  400<» ;  —  .  —  .—,  same  after  accamala- 

tion  of  sediments. 


have  strata  20,000  and  40,000  and  even  more  feet  thick. 
The  lower  portions  of  such  strata  must  be  completely 
metamorphic.  The  figure  (Fig.  134)  shows  how  the  pro- 
cess takes  place. 

Crushing-. — Pressure  alone  is  a  condition,  but  not  a 
cause  of  heat.  But  pressure  producing  motio7i,  or  crush- 
ing, crumpling,  is  an  acMve  cause  of  heat.  Now,  we 
usually  find  metamorphism  associated  with  most  complex 
crumpling  of  strata.  The  heat  must  have  been  increased 
also  by  this  cause. 

Even  igneous  rocks,  by  pressure,  mashing,  and  shear- 
ing, may  be  made  to  assume  the  appearance  of  metamor- 
phic stratified.  Many  schists,  especially  gneisses,  are 
formed  in  this  way.. 


CHAPTER  V. 

STRUCTUKES   COMMON  TO   ALL  ROCKS. 

We  have  now  given  a  brief  account  of  all  the  different 
kinds  of  rocks.  But  there  are  still  some  structures  which 
are  found  in  all  kinds  of  rocks,  and  which  could  not  be 
described  until  these  kinds  had  been  defined.  These  are: 
1.  Joints;  2.  Great  fissures;  and,  3.  Mineral  veins. 
Mountain-chains,  as  involving  all  kinds  of  rocks  and  all 
kinds  of  structure — in  fact,  as  summing  up  all  the  prin- 
ciples of  dynamical  and  structural  geology — we  must  take 
last  of  all. 

Sectiok  I Joints  akd  Fissures. 

Joints, 

"We  have  already  alluded  to  joints  in  stratified  rbcks 
(page  179),  but  without  describing  them,  because  not 
characteristic  of  these  rocks.  All  rocks — sedimentary, 
igneous,  and  metamorphic — are  divided  by  cracks  in  dif- 
ferent directions  into  separable  blocks  of  various  sizes  and 
shapes.  These  cracks  are  called  joints.  In  stratified 
rocks,  one  of  the  division-planes  is  between  the  strata,  and 
the  other  two  nearly  at  right  angles  to  this.  The  shape  and 
size  of  the  blocks  difier  in  different  kinds  of  rocks.  For 
example,  in  sandstone  the  blocks  are  usually  very  large 
and  roughly  prismatic  ;  in  limestone,  they  are  usually 
very  regularly  cubic  (Fig.  135)  ;  in  shale,  oblong  rhom- 
boidal ;  in  slate,  small  and  sharply  rhombic  ;  in  granite, 
sometimes  large  and  roughly  cubic,  sometimes  scaling  in 
228 


STRUCTURES  COMMON  TO  ALL  ROCKS.        229 

concentric  shells,  producing  domes  ;  in  eruptives,  of  many 
shapes,  rough  cubic,  ball-like,  regular  columnar,  tilelike. 


Fig.  135.— Regular  jointing  of  limestone. 

For  this  reason  a  cliff,  especially  of  stratified  rock,  looks 
like  a  wall  of  titanic  masonry  without  mortar. 

Cause. — These  cracks  are  supposed  to  have  been  formed 
by  the  shrinkage  of  the  rocks  ;  in  stratified  rocks,  in  con- 
solidating from  sediments  ;  in  igneous  and  metamorphic 
rocks,  in  cooling  from  a  state  of  fusion  or  semi-fusion. 
In  stratified  rocks  they  are  usually  confined  to  the  stra- 
tum, though  some  larger  Joints  (master- joints)  run 
through  several  strata.  They  are  mentioned  mainly  that 
the  student  should  not  confound  them  with  other  kinds 
of  structure. 

Great  Fissures. 

Joints  are  probably  shrinkage-cracks.  Fissures  are 
fractures  by  crust-movements.  Joints  are  cracks  of  the 
individual  strata  ;  fissures  are  fractures  of  the  earth's 
crust,  extending  through  many  formations,  and  continu- 
ing for  many  miles. 

Cause. — We  shall  see  hereafter  that  the  earth's  crust 
is  subjected  to  a  powerful  horizontal  pressure,  by  which 


230 


STR UCTURAL   QEOLOQ  Y. 


it  is  sometimes  mashed  together,  sometimes  thrown  into 
arches  and  hollows.  Such  bendings  of  the  crust  produce 
enormous  fractures  parallel  to  the  axis  of  the  bending, 
and  parallel  to  mountain-ranges,  since  mountain-ranges 
are  produced  in  this  way.  Sometimes  there  is  a  system 
at  right  angles  to  the  main  system,  or  in  the  direction  of 
the  cross-valleys  of  mountains. 

The  characteristics,  therefore,  of  great  fissures  are — 
1.  Their  occurrence  in  systems,  usually  parallel  to  the 
axis  of  elevation.  2.  Their  length,  often  extending  for 
hundreds  of  miles.  3.  Their  depth,  sometimes  breaking 
through  miles  of  thickness  of  rock.  When  filled  at  the 
moment  of  formation  with  fused  matter  from  below,  they 


Fia.  136.— Fault  in  Southwest  Virginia  :  a,  Silurian  :  «i,  carboniferous.  (After  Lesley.) 


form  dikes ;  and  all  great  dikes  and  igneous  overflows 
have  been  through  such  fissures.  But  if  not  filled  at  once 
with  fused  matter,  but  slowly  afterward  with  mineral 
matter,  they  form  the  gv^^Li  fissure-veins.  Whether  they 
are  filled  at  once  with  fused  matter,  or  afterward  slowly 
with  mineral  matter,  or  remain  empty,  the  walls  do  not 
usually  remain  in  their  original  position,  but  nearly  always 
slip  one  on  the  other  up  or  down.  Such  a  displacement 
of  the  crust  on  the  two  sides  of  a  fissure  is  called  a  fault. 
We  have  already  treated  of  dikes  ;  we  shall  hereafter  take 
up  mineral  veins.     We  must  now  speak  briefly  of  faults. 

Faults. 

As  already  explained,  these  are  displacements  of  fissure- 
walls.     They  take  place  on    an   immense  scale.     Lesley 


STRUCTURES  COMMON  TO  ALL  ROCKS.        231 


mentions  a  fissure  in  Pennsyl- 
vania in  which  the  vertical  dis- 
placement is  20,000  feet,  and 
may  be  traced  for  twenty  miles. 
Rogers  describes  one  in  southern 
Virginia  in  which  the  displace- 
ment is  8,000  feet,  and  may  be 
traced  for  eighty  miles  (Fig. 
136). 

According  to  Powell,  there  is 
on  the  north  side  of  the  Uintah 
Mountains  a  vertical  slip  of 
20, 000  feet.  All  along  the  east- 
ern side  of  the  Sierra  there  is  a 
slip  of  not  less  than  15,000  to 
20,000  feet ;  and  King  thinks 
the  slip  on  the  west  side  of  the 
Wahsatch  is  even  40,000  feet. 
But  they  are  developed  on  per- 
haps the-  grandest  scale  in  the 
Colorado  plateau  region.  This 
high  plateau  is  traversed  by  a 
system  of  north  and  south  fis- 
sures, 100  to  200  miles  long,  by 
which  the  arched  earth-crust  is 
broken  into  huge  blocks,  and 
these  have  settled  to  different 
levels,  some  5,000  to  12,000  feet 
below  others,  and  thus  give  rise 
to  a  wonderful  system  of  north 
and  south  cliffs  (Fig.  137). 

If  such  a  vslip  takes  place  sud- 
denly, then  at  first  tliere  must 
have  been  a  cliff  as  great  as  the 
slip.  The  same  would  be  true 
even  with  gradual  slipping,  if 


l^\ 


ij  i^M 


VII 


.e  i 


I  'V  i 


232  STRUCTURAL  GEOLOGY. 

there  were  no  erosion.  But  both  the  slipping  and  the 
erosion  have  probably  been  going  on  slowly  all  the  time, 
and  whether  there  be  a  cliff  or  not,  depends  on  the  age 
of  the  fracture  and  the  relative  rate  of  slipping  and  ero- 
sion. In  many  of  the  faults  of  the  plateau  and  basin 
region,  the  cliff  still  exists  (though  not  as  great  as  the  dis- 
placement), because  of  the  comparative  recency  of  the 
fractures  and  dryness  of  the  climate.  The  great  Sierra 
fault  is  marked  by  a  steep  slope  of  8,000  to  10,000  feet 
to  the  east,  that  of  the  Wahsatch  of  8,000  feet  to  the 
west.  But  in  the  Uintah  fault,  and  in  all  the  faults  of 
the  Appalachian  region,  there  is  actually  no  surface-sign 
of  the  fault  (Fig.  136).  We  may  stand  astride  of  the 
fissure. 

Two  Kinds  of  Faults. — Faults  are  of  two  kinds,  ac- 
cording to  the  direction  of  the  slip.  Fissures  are  nearly 
always  iriclined,  and  therefore  have  what  miners  call  a 
foot  (lower)  wall  and  a  hanging  (upper)  wall.  More  com- 
monly the  hanging  wall  drops  down.  These  are  normal 
faults.  But  sometimes  the  hanging  wall  is  pushed  up 
over  the  foot  wall.  These  are  called  reverse  faults.  In 
normal  faults  the  broken  parts  are  readjusted  by  gravity 
(settling)  ;  in  reverse  faults  the  broken  parts  are  crushed 
together  and  forced  to  slide.     The  upper  wall  over  the 


Pig.  138.— Section  across  Yarrow  Colliery,  showing  the  law  of  faults.    (After 
De  la  Beche.) 


lower  (Fig.  136)  is  a  reverse  fault.     Fig.  136  shows  only 
normal  faults. 

Law  of  Slip. — In  cases  of  displacement  of   strata  it 
becomes  often  a  matter  of  great  importance,  not  only  to 


r 


STRUCTURES  C03IM0N  TO  ALL  ROCKS,        233 

the  field  geologist  but  also  to  the  practical  miner,  to  know 
which  side  has  gone  up  or  down  ;  for  valuable  beds  of 
coal  or  veins  of  metal  are  thus  displaced,  and  it  is  impor- 
tant to  know  which  way  they  went.  Now,  normal  faults 
are  far  the  more  common,  and  therefore  a  very  general 
though  not  universal  rule  in  such  cases  is  this  :  In  case 
of  inclined  fissures,  the  foot-wall  or  lower  side  has  gone 
up,  or  the  hanging  wall  or  upper  side  has  dropped  down. 
Or,  it  may  be  otherwise  expressed,  thus:  *^The  dip  or 
hade  (slope)  of  the  fissure  is  toward  the  down-throw.'* 
In  Fig.  138,  which  represents  an  actual  section,  the  rule 
is  followed  in  every  fissure.  The  exceptions  to  this  rule 
(Fig.  136)  are  found  only  in  the  strongly  folded  rocks  of 
mountain-regions. 

Section  II. — Mineral  Veins. 

Let  any  one  examine  rocks,  especially  metamorphio 
rocks,  in  mountain-regions,  and  he  will  see  that  they  are 
marked  with  seams  and  scars  running  in  all  directions,  as 
if  they  had  been  crushed  and  broken  and  again  mended  ; 
as  indeed  they  were.  Now,  all  such  markings  and  seam- 
ings,  whatever  be  their  nature  and  origin,  are  often  called 
by  the  general  name  of  veins.  Thus,  beds  of  coal,  or 
gypsum,  or  salt,  on  the  one  hand,  and  the  fillings  of 
fissures  by  fused  matter,  on  the  other,  are  sometimes 
called  veins.  It  is  evident  that  no  scientific  progress  can 
be  made  so  long  as  things  so  different  are  confounded 
under  the  same  name. 

Definitiou. — Putting  aside,  then,  all  beds  formed  aa 
sediments  at  the  bottom  of  water,  such  as  coal,  gypsum, 
etc.,  and  all  fillings  of  fissures  by  fused  matter,  such  as 
dikes,  etc.,  veins  may  be  defined  as  (usually)  the  fillings 
of  fissures  or  cracks  by  slow  deposits  from  solution  in  per- 
colating waters,  of  materials  leached  from  the  surround- 
ing or  underlying  rocks.     Since  the  deposit  takes  place 


234  STRUCTURAL  GEOLOOY. 

from  solution,  the  materials  of  veins  are  in  a  purer  and 
more  sparry  condition  than  they  exist  in  the  rocks. 

It  is  evident  that,  as  thus  defined,  veins  must  vary 
greatly  in  appearance.  Sometimes  they  are  fine  lines,  the 
fillings  of  small  cracks  produced  by  rock-crushing.  Some- 
times they  are  the  fillings  of  larger  joints.  Sometimes 
they  are  the  fillings  of  great  fissures  breaking  through  the 
earth-crust.  It  is  these  last  which  are  far  the  most  im- 
portant ;  and  it  is  only  on  these,  therefore,  that  we  shall 
dwell. 

Fissure-Veins. — As  these  are  the  fillings  of  those 
great  fissures  which  are  formed  by  crust-movements, 
they  are  of  great  extent.  The  fillings  of  such  fissures  at 
once  with  fused  matters  are  called  dikes  (page  216)  ;  the 
fillings  by  slow  deposit  of  mineral  matter  ^vq  fissure-veins. 
These  veins,  therefore,  like  fissures  (page  229),  of  which 
they  are  the  fillings,  are  often  many  miles  in  extent, 
many  feet  in  width,  and  of  unknown  but  certainly  many 
thousand  feet  in  depth.  Like  fissures,  the}?-  occur  in  sys- 
tems, parallel  to  each  other  and  to  the  axis  of  elevation 
of  the  mountain  where  they  occur.  Between  the  vein 
and  the  wall-rock  on  either  side  there  commonly  exists  a 
layer  of  clay  called  the  selvage.  It  is  very  characteristic 
of  true  fissure-veins,  and  probably  produced  by  the  solvent 
effect  on  the  wall-rock  of  water  circulating  between  the 
vein  and  the  wall. 

Metalliferous  Veins. — Metals  may  occur  in  beds,  for 
example,  iron  (page  299),  or  filling  cavities  of  any  kind 
in  rocks,  as  sometimes  lead.  But  they  most  commonly 
occur  in  veins,  especially  fissure-veins.  The  further  de- 
scription of  fissure-veins  is  best  undertaken,  therefore, 
under  this  head. 

Contents. — AVe  must  not  imagine  that  metalliferous 
veins  are  filled  with  metals.  The  fillings  of  fissures  are  of 
two  kinds,  viz.,  the  vein-stuff,  vein  rock,  gangue,  or  matrix 
(as  it  is  variously  called),  and  the  metallic  ore.     By  far 


STRUCTURES  COMMON  TO  ALL  ROCKS,        235 

the  larger  portion  is  usually  vein-stuff  ;  and  through  this 
is  disseminated  the  metallic  ore  in  granules,  strings,  or 
larger  masses  (Fig.  140,  c),  or  sometimes  in  a  central  sheet 
(Figs.  139,  141),  as  if  de- 
posited last  of  all.  The 
principal  ki7ids  of  vein-stuffs 
are  silica,  carbonates  of  lime, 
iron  and  baryta,  sulphate  of 
baryta,  and  fluoride  of  cal- 
cium (fluor-spar).  Often, 
however,  many  kinds  of 
minerals  are  aggregated  into 
a  veritable  vein-rock.  The  most  common  of  all  is  silica, 
in  the  form  of  quartz.  Next  comes  lime-carbonate.  The 
metals  sometimes  occur  free  (M),  as,  for  example,  always 
gold  and  platinum,  often  silver,  and  sometimes  copper  and 
mercury.  But  more  commonly  they  occur  as  metallic 
sulphides  (MS),  carbonates  (MCOg),  and  oxides  (MO). 
By  far  the  most  common  form  is  sulphides.  These  facts 
are  given  in  the  schedule.  The  most  abundant  kinds  are 
marked  with  a  + . 

Structure. — Veins  have  nearly  always  a  more  or  less 
banded  structure,  as  if  the  materials  were  deposited  in 
successive  layers,  on  the  two  sides  alike.     Sometimes  the 


VEIN-STUFF. 

ORE. 

+  +   SiOa 

+  +   MS 

+   CaCOs 

+    MCOa 

F'COa 

MO 

BaCOa 

M 

BaS04 

CaFl 

hah 


b     a   c 


Fig.  139. — a,  central  sheet  of  ore ; 
66,  agate  ;  rf,  wall-rock. 


Fig.  140.— aa,  agate  ;  6,  quartz ;  c, 
copper-bearing  lode  ;  tf,  wall-rock. 


successive  layers  are  of  the  same  material,  but  of  different 
colors  (Figs.  139,  Ih,  140,  aa)',  sometimes  of  different  ma- 


236 


STRUCTURAL   OEOLOOY. 


be  babe  h 


d 


terial  (Fig.  141).  Sometimes  the  bands  are  beautifully 
regular  and  distinct,  like  agate  (Figs.  139,  140)  ;  some- 
times on  a  larger  scale,  and  irregular.  Very  often  we  find 
several  corresponding  layers  of 
agate  on  the  two  sides,  and  the 
center  filled  with  combs  of  quartz- 
crystals  with  interlocking  teeth 
(Fig.  140,  I). 

Irregularities. — Small  veins, 
the  fillings  of  small  cracks,  are 
extremely  irregular,  running  in 
all  directions,  and  intersecting 
each  other  in  every  conceivable  way.  Great  fissure-veins 
are  far  more  regular.  But  even  these  are  more  or  less 
irregular,  partly  from  the  irregularity  of  the  original  fis- 
sure and  partly  from  subsequent  movements.  Perhaps 
the  most  important  of  these  is  displacements  by  fissures 
or  other  veins,  as  explained  below. 

Ag-e  of  Veins. — Often  in  the  same  locality  we  find 
two  or  more  systems  of  veins,  formed  at  different  times, 
crossing  each  other.  In  such  cases,  as  in  dikes,  the  fissure 
or  vein  which  cuts  through  the  other  (Fig.  142,  a)  is  of 


Fig.  141.— a,  galena ;  66,  bary 
ta  ;  cc,  fluor-spar  ;  c?,  wall. 


Fig.  142. 


course  the  younger.  The  absolute  age,  i.  e.,  the  geological 
period  in  which  the  fissure  was  made,  can  be  known  only 
by  the  age  of  the  strata  through  which  it  breaks. 

Recovery  of  Lost  Veins. — Suppose  Z>  (Fig.  142)  is  a 
valuable  vein,  and  we  work  down  until  we  strike  a.     The 


STRUCTURES  COMMON  TO  ALL  ROCKS.        237 

vein  is  here  lost  by  slips  ;  which  way  shall  we  go  to  recover 
it  ?  Remember  the  rule  already  given  on  page  233  :  ^^  The 
slope  or  dip  or  '  hade '  of  the  displacing  fissure  (here  «)  is 
toward  the  down-throw."  This  rule  is  not  invariable,  but 
very  general. 

Surface-Changes. — We  must  not  imagine  that  metal- 
liferous veins  outcrop  on  the  surface  in  the  form  we  have 
described.  If  they  contain  no  metal,  veins  may  indeed 
appear  unchanged  on  the  surface.  Quartz-veins  may,  for 
example,  be  often  traced  over  hillsides  by  strewed  frag- 
ments of  white  quartz.  But  metalliferous  veins  are  usually 
so  greatly  changed  on  the  surface  that,  without  much  ex- 
perience, we  would  not  recognize  them  at  all.  Precisely 
as  rocks  are  usually  concealed  by  soil  resulting  from  sur- 
face-decomposition, so  veins  are  concealed  by  surface- 
changes.  To  the  experienced  eye  these  surface-changes 
become  surface-^i'^/is,  and  are  therefore  of  the  greatest 
practical  importance.  These  surface-signs  are  far  too 
complex  and  various  to  be  explained  here.  We  only 
mention  them  to  guard  the  pupil  against  supposing  that 
it  is  easy  to  see  what  we  have  described  above,  and  to 
stimulate  him  to  observe  for  himself. 

Orig-in  of  Mineral  Veins. — ^This  is  a  difficult  and 
obscure  subject,  but  the  following  propositions  are  prob- 
ably true  :  1.  Veins  have  been  formed  by  deposit  of  min- 
eral matters  from  solutions  in  percolating  or  subterranean 
waters.  2.  The  movement  of  the  subterranean  waters  may 
have  been  in  any  direction,  but  mostly  up-coming.  3. 
The  waters  may  have  been  at  any  temperature,  but  mostly 
liot.  4.  The  water-ways  may  have  been  of  any  kind,  bat 
the  openest  water-ways — the  highways  of  ascending  waters 
— are  open  fissures.  5.  The  waters  have  been  usually, 
though  perhaps  not  always,  alkaline,  i.  e.,  containing 
alkaline  carbonates  or  alkaline  sulphides,  or  both. 


238  '  STRUCTURAL   GEOLOGY, 


Section    III. — Mountains  :    their  Origin  and 
Structure. 

Mountains  are  the  glory  of  the  earth — the  culminating 
points  of  scenic  grandeur  and  beauty.  But  few  perceive 
that  they  are  so  only  because  they  are  also  the  culmi- 
nating points  of  all  geological  agencies.  This  is  but 
one  illustration  of  the  general  truths  that  there  is  an  in- 
dissoluble and  necessary  connection  between  truth  and 
beauty,  between  science  and  fine  art.  It  is  evident,  then, 
that  the  study  of  mountains  is  the  key  to  dynamical  and 
structural  geology. 

The  difficulty  which  meets  us  at  tlie  threshold  of  this 
subject  is  the  loose  use  of  the  term  mou7itain.  The  term 
is  used  to  express  every  conspicuous  elevation  above  the 
general  level,  whatever  be  its  extent,  and  in  whatever  way 
it  may  have  been  formed.  Thus  an  isolated  eminence 
produced  by  circum-erosion,  or  a  peak  formed  by  volcanic 
ejection,  a  ridge  between  two  stream-gorges,  a  great  bulge 
produced  by  the  folding  of  the  earth^s  crust,  or  a  series  of 
such  foldings  parallel  to  each  other  in  the  same  general 
region — are  all  called  by  the  same  name,  mountain. 
Qualifying  terms  are  indeed  often  used,  such  as  moun- 
i'ain-peah,  mountain-no^^e,  mountain-ra?z^e,  etc.,  but 
these  also  are  used  loosely  and  interchangeably.  It  is 
necessary,  therefore,  first  of  all,  to  define  our  terms. 

Deftnitious — A  mountain-c/^«?'?i,  or,  better,  moun- 
tain-system, is  an  assemblage  of  ranges  parallel  to  each 
other  in  the  same  general  region,  but  usually  formed  at 
different  times  {poly(fe7ietic).  All  the  great  mountain- 
chains  of  the  world  are  of  this  nature.  For  example  : 
The  Appalachian  system  consists  of  the  Blue  Ridge,  the 
Alleghany,  and  the  Cumberland  ranges.  The  Rocky 
Mountain  system,  or  North  American  Cordilleras,  consists 
01  the   Colorado  range,  the  Park  range,  the  Wahsatch^ 


STRUCTURES  COMMON  TO  ALL  ROCKS.        239 

the  Sierra,  the  Coast  ranges,  and  many  others.  So  the 
Alps,  the  Himalayas,  and  the  Andes  consist  also  of  sev- 
eral parallel  ranges. 

A  moiintaiii-raii^e  is  one  of  these  great  components, 
formed  at  one  time — by  one  earth-effort  {mo )io genetic), 
though  the  effort  may  have  continued  through  a  great 
period  of  time.  The  Colorado,  the  Uintah,  the  Wah- 
satch,  the  Sierra,  and  the  Coast  ranges  are  good  examples. 
The  Blue  Kidge  and  the  Alleghany  ranges  are  also  good 
examples. 

A  iiiountain-ridge  is  a  subdivision,  again,  of  a  range, 
produced  usually  by  erosion,  although  sometimes  also  by 
foldings  of  strata.  Mountain-pea hs  are  serrations  of  the 
crest  of  a  range  or  a  ridge,  either  by  erosion  or  by  volcanic 
ejections. 

Mountain-systems  are  separated  by  great  interior  conti- 
nental basins  ;  mountain-ranges  by  great  valleys  j  moun- 
tain ridges  and  peaks  by  narrow  valleys  or  gorges. 

Such  is  the  simplest  view  of  the  form  of  mountains ; 
but  sometimes  a  mountain-range  seems  to  be  composed  of 
an  inextricable  tangle  of  ridges  running  in  all  directions. 

Now,  it  is  evident  that  any  scientific  discussion  of  the 
origin  and  structure  of  mountains  must  be  essentially  that 
of  the  origin  and  structure  of  ranges;  for,  on  the  one 
hand,  a  mountain-system  is  a  mere  adding  of  range  to 
range,  and,  on  the  other,  ridges  and  peaks  are  the  result 
of  subsequent  sculpturing  by  rain  and  rivers.  It  is  of 
ranges,  therefore,  that  we  shall  mainly  speak. 

The  surface  of  the  earth  has  now  become  cool  and  its 
mean  temperature  fixed,  and  is,  therefore,  no  longer  con- 
tracting J  but  the  interior  is  certainly  still  extremely  hot, 
and  still  cooling  and  contracting.  The  effect  of  such 
interior  contraction  is  to  thrust  the  exterior  crust  upon 
itself  horizontally  with  irresistible  force,  crushing  it  to- 
gether with  many  complex  foldings  of  strata,  and  caus- 
ing it  to  bulge  up  in  long  wrinkles.     Such  lines  of  bulging 


240 


STRUCTURAL   OEOLOOY. 


or  wrinkles  are  mountain-ranges.  So  much  it  was  neces- 
sary to  say  to  render  what  follows  intelligible  ;  but  the 
origin  of  mountains  is  best  taken  up  in  connection  with 
their  structure. 

Structure  and  Origin  of  Mountains. 

Mountain-ranges  are  always  made  up  of  series  of  strata 
of  immense  thickness  thrown  into  folds,  as  if  they  had 
been  crushed  together  horizontally,  and  swelled  up  verti- 


PiG.  143. 

cally.  To  illustrate  :  Suppose  we  had  a  number  of  layers 
of  wax,  or  clay,  or  other  plastic  substance  of  different 
colors  laid  one  atop  another,  as  in  Fig.  143,  A  ;  suppose, 
further,  that  the  middle  portions  were  softened  a  very 
little  by  gentle  heat  below,  and  the  whole  was  then 
crushed  together  horizontally,  as  represented  by  the 
arrows.     The  middle  softer  portions  would  yield  and  be 


Fig.  144.— Section  across  the  Uintah. 


mashed  together,  thrown  into  folds  and  swelled  up,  as 
shown  in  Fig.  143,  B.     Now,  this  is  exactly  the  way  in 


STRUCTURES  COMMON  TO  ALL  ROCKS.        *^41 

^'hich  mountain-ranges  seem  to  have  been  formed.  Some- 
times, though  rarely,  there  is  but  one  great  fold  (Fig. 
144) ;  sometimes  there  are  several  open  folds  (Fig.  145)  ; 


Pig.  145.— Section  across  the  Jura. 

more  commonly,  especially  in  great  mountains,  there  are 
many  closely  appressed  folds  (Figs.  14G  and  147).  In 
the  Coast  Hiinge  (Fig.  146)  there  are  at  least  five  alternate 
anticlines  and  synclines ;  in  the  Alps  there  are,  in  some 
places,  seven  alternate  anticlines  and  synclines.     It  is 


Pia.  146.— Section  of  Coast  Range,  showing  plication  by  horizontal  pressure. 

evident  that  in  these  cases  a  great  breadth  of  sediments 
is  squeezed  horizontally  into  a  small  space,  and  corre- 
spondingly swelled  upward  into  a  range.  In  the  case  of 
the  Coast  Eange  (Fig.  146),  every  two  or  two  and  a  half 
miles  of  original  breadth  has  been  compressed  into  one 


Fig.  147.— Appalachian  chain. 


mile.  In  the  case  of  the  Alps,  probably  every  three  miles 
of  original  breadth  has  been  crushed  into  one  mile,  and, 
of  course,  correspondingly  swelled  up.  Sometimes  the 
mashing  together  is  even  far  greater  than  represented  in 


Le  Conte,  Geol.  16 


242 


STRUCTURAL   OEOLOOY. 


these  figures.     Fig.  148  shows  an  example  in  the  Alps, 
taken  from  Heim. 

There  is  another  evidence  that  mountains  are  formed 
wholly  by  horizontal  crushing,  viz.,  the  phenomenon  oi 
slaty  cleavage.     We  have  already  seen  (page  105)  that  slaty 


Fig.  148.— Section  across  central  Alpe  :  J,  Jurassic  ;  ^  triassic  ;  s,  schist. 


cleavage  always  shows  a  crushing  together  horizontally, 
and  an  extension  vertically,  of  the  whole  mass.  Now, 
cleavage  is  always  associated  with  folded  strata  and  with 
mountain-ranges. 

Mountains  are  often  spoken  of  as  due  to  *'  upheaval ." 
There  is  no  objection  to  the  use  of  this  term,  if  it  be  re- 
membered that  the  upheaval  is  not  usually  due  to  a  force 
VLCting  from  helow  upwar'dj  but  to  a  horizontal  force  crush- 
ing together  and  swelling  upward  by  thickening  the  whole 
squeezed  mass. 

We  have,  in  Fig.  143,  B,  given  the  ideal  structure  of  a 
mountain-range  if  there  had  been  no  erosion.  But,  of 
course,  as  soon  as  a  mountain  begins  to  rise,  rain-ioater 
begins  to  cut  it  aumy,  and  in  all  mountains  the  amount 
cut  away  is  immense,  in  many  far  greater  than  what  is 
left.  This  fact  is  represented  in  the  preceding  figures 
(144-148).  In  all  these  figures,  however,  except  the  last, 
the  range  is  composed  wholly  of  stratified  rock  ;  but  in 
most  great  mountain -ranges  we  have  an  axis  of  crystalline 


STRUCTURES  COMMON  TO  ALL  ROCKS.        243 

rock,   granitic  or  metamorphic,   flanked   on  either   side 
with  uptilted  and  folded  strata,  as  in  Figs.  149,  150.     It 


Fig.  149.— Ideal  section,  showing  granite  axis. 

was  formerly  supposed  that  the  igneous  rock  in  fused  con- 
dition has  pushed  up  and  broken  through  the  strata  and 
appeared  above  them.  But  it  is  far  more  probable  that 
stratified  rock  once  covered  the  whole,  as  shown  by  the 
dotted  lines,  and  that  subsequent  erosion  has  exposed  the 


Fig.  150.— Ideal  section  of  a  mountain-range. 

granitic  or  metamorphic  rocks  along  the  crest  where  the 
erosion  was  greatest.  Furthermore,  when  we  remember 
that  mountains  are  composed  of  immensely  thick  series 
of  strata,  and  that  very  thick  strata  are  sure  to  be  meta- 
morphic in  their  lower  parts  (page  226),  and,  moreover, 
that  granite  is  often  but  the  last  term  of  metamorphism 
of  rocks,  it  becomes  probable  that  even  such  mountains 
as  those  represented  in  Figs.  149,  150,  are  really  com- 
posed wholly  of  horizontally  mashed  and  crumpled  strata, 
only  that,  on  account  of  the  great  thickness  and  strong 
crumplings,  these  have  become  completely  metamorphic 
in  their  lower  parts. 


244  STRUCTURAL  GEOLOGY. 

Thickness  of  Mountain  Sediments. — We  have  said 
that  mountains  are  composed  of  enormously  thick  sedi- 
ments, crushed  together  horizontally  with  many  crump- 
lings,  and  swelled  up  proportionally.  We  will  now  give 
examples  of  such  thickness.  The  Appalachian  consists 
of  folded  strata  (Fig.  147)  which,  according  to  Hall,  are 
not  less  than  40,000  feet,  or  nearly  eight  miles,  in  thick- 
ness. The  Wahsatch  consists  of  sediments  which,  ac- 
cording to  King,  are  50,000  feet,  or  nearly  ten  miles,  in 
thickness.  The  Coast  Eange  of  California  consists  of 
folded  cretaceous  and  tertiary.  The  cretaceous  alone, 
according  to  Whitney,  are  20,000  feet  thick.  The  tertiary 
have  not  been  measured,  but  cannot  be  less  than  10,000 
feet.  So  that  at  least  30,000^  feet,  or  nearly  six  miles, 
thickness  of  sediments  are  involved  in  the  folded  struc- 
ture of  this  range  (Fig.  146).  The  strata  of  the  Alps  are 
not  less  than  40,000  to  50,000  feet  thick.  The  same  is 
true  of  all  mountains. 

Now,  we  must  not  imagine  that  this  is  evidence  of  the 
average  thickness  of  strata,  but  only  revealed  in  moun- 
tains by  erosion,  for  the  very  same  strata  elsewhere  are 
much  thinner.  For  example,  the  same  strata,  which  are 
40,000  feet  thick  in  the  Appalachian  Eange,  thin  out  west- 
ward until  they  are  only  4,000  feet  thick  at  the  Mississippi 
Kiver.  The  very  same  strata,  which  are  30,000  feet  thick, 
in  the  Wahsatch,  thin  out  eastward,  and  are  only  1,000 
feet  thick  on  the  Plains.  Thus,  then,  mountain-ranges, 
hefore  they  were  upheaved,  were  lines  of  exceptio7ially  thick 
sediments.     This  may  be  regarded  as  certain. 

Mountain-Ranges  are  Upheaved  Marginal  Sea- 
Bottoms. — Where,  then,  do  we  find  exceptionally  thick 
sediments  ?  Where,  but  along  marginal  sea-bottoms  ? 
We  have  seen  (page  48)  that  here  are  accumulated  nearly 
the  whole  debris  of  continental  erosion.  Therefore, 
mountaln-ranqes  hefore  they  were  upheaved,  were  mar- 
ginal sea-bottoms  on  lohich  have  accumulated  enormously 


STRUCTURES  COMMON  TO  ALL  ROCKS.        245 

thich  sediments.  Every  one  of  our  great  mountain-ranges 
can  be  shown  by  geological  evidence,  which  we  cannot 
give  here,  to  have  occupied  this  position  until  the  time 
of  their  birth.* 

Difterent  Stages  of  Mountain-Life. — We  have  said 
"  until  birth"  but  it  must  not  be  supposed  that  there  was 
anything  sudden  about  it.  The  emergence  above  water 
we  call  its  hirth,  but  a  mountain  continues  to  grow  stead- 
ily through  many  ages.  Meanwhile,  as  soon  as  it  is  born, 
erosion  commences,  and  continues  with  increasing  rate  as 
the  range  grows  higher.  When  the  mountain  stops  grow- 
ing, erosion  begins  to  destroy  it,  and  finally  levels  it  com- 
pletely. Thus,  in  every  mountain  there  is  a  period  of 
birth,  a  period  of  growth,  a  period  of  maturity,  a  period 
of  decay,  and  a  time  of  death  or  obliteration.  Many  of 
the  earliest  mountains  have  been  entirely  swept  away. 
We  know  their  places  only  by  their  folded  structure — 
fossil  bones  of  extinct  mountains. 

Why  Yielding  occurs  along  Lines  of  Thick  Sedi- 
ments.— Perhaps  the  pupil  has  already  asked  himself, 
^^  Why  does  yielding  occur  only  along  lines  of  thick  sedi- 
ments ? "  The  probable  reason  is,  that  great  accumu- 
lations cause  the  rise  of  the  interior  heat  of  the  earth 
toward  the  surface,  as  already  explained  on  page  227. 
This  heat,  in  the  presence  of  the  water  included  in  the 
sediments,  causes  these,  as  also  the  earth-crust  beneath, 
to  soften  or  even  semi-fuse  ;  and  thus  creates  a  line  of 
iveahness,  and  therefore  of  yielding.  This  is  represented 
in  the  experiment  with  the  wax,  on  page  240,  by  the  gen- 
tle softening  of  the  middle  part. 

Cause  of  the  Lateral  Pressure. — If  it  be  further 
asked,  "  What  is  the  cause  of  the  lateral  pressure  ?'^  we 
can  only  say  that  this  is  an  obscure  point,  and  one  much 
discussed.  It  is  probable,  however,  that  it  is  due,  as 
already  stated  (page  239),  to  the  interior  contraction  of 
*  For  evidence,  see  "  Elements  of  Geology,"  p.  265. 


246  STRUCTURAL  GEOLOGY, 

the  earth,  by  which  the  crust,  following  down  ths  shrink- 
ing nucleus,  is  thrust  upon  itself  laterally  with  irresist- 
ible force.     Mountain-ranges  are  the  lines  of  yielding. 

Other  Associated  Plienonieiia. — If  we  clearly  appre- 
hend the  foregoing  account  of  the  structure  and  origin 
of  mountains,  other  associated  j^henomena  are  easily  un- 
derstood :  1.  The  strong  bendings  of  the  strata  neces- 
sarily produce  fissures,  mainly  parallel  to  the  bendings — 
i.e.,  to  the  axis  of  the  range,  and  to  one  another.  2. 
Since  these  fissures  break  through  many  miles  of  strata, 
it  is  natural  that  igneous  matter  should  come  up  through 
them  to  the  surface,  and  therefore  that  volcanic  and 
especially  gvQ2i.i  fissure  eruptions  should  be  associated  with 
mountain-ranges,  and  that  where  the  overflows  are  cut 
away  by  erosion  we  should  find  ilihes.  Again,  3.  As  the 
mashing  goes  on  steadily,  the  fissures  first  formed  would 
be  certain  to  slip,  and  thus  we  find  great  faults  often  as- 
sociated with  mountains.  Again,  4.  The  formation  of  a 
fissure  or  the  subsequent  slipping  of  a  fissure  could  not 
fail  to  produce  an  earth- jar ;  and  thus  earthquakes  are 
commonest  in  mountain-regions.  Finally,  5.  Fissures 
which  did  not  fill  at  the  moment  of  formation  by  igneous 
injection  would  certainly  fill  slowly  afterward  by  perco- 
lating water  depositing  minerals,  and  thus,  also,  mineral 
veins  are  commonest  in  mountain-regions. 

Thus  we  see  now  the  truth  of  the  proposition  with 
which  we  set  out,  viz.,  that  mountains  are  the  culminat- 
ing points — the  theaters  of  greatest  activity — of  all  geo- 
logical agencies ;  of  aqueous  sedimentary  agencies  in 
preparation  for  the  mountain  ;  of  igneous  agencies  in 
the  birth  and  growth,  and  of  aqueous  erosive  agencies 
in  sculpturing  and  final  destruction  of  the  mountain. 

Mountain- Sculpture, 

In  the  lifcrhistory  of  a  mountain-range,  the  work  of 
water  in  sculpturing  is  no  less  important  than  the  work 


STRUCTURES  COMMON  TO  ALL  ROCKS.        247 

of  interior  heat  in  formatio7i.  If  the  mountain  is  rough- 
hewed  by  the  latter,  it  is  shaped  and  chiseled  by  the 
former,  The  great  swell  of  the  crust,  which  is  only  seen 
from  a  distance,  is  due  to  igneous  agency  ;  but  all  the 
scenery,  which  so  charms  us  when  we  are  amo7ig  moun- 
tains, is  due  wholly  to  erosion.  Moreover,  there  is  a 
peculiar  charm  in  the  study  of  the  latter,  because  it  is 
more  easily  understood.  The  cause  of  mountain-origin 
is  obscure,  and  the  folded  structure  of  mountains  is 
hidden,  and  can  only  be  unraveled  by  the  skillful  geol- 
ogist ;  but  the  forms  of  mountain-sculpture  may  be 
studied  by  all,  and  their  study  gives  great  additional 
charm  to  mountain-travel. 

Resulting  Forms. — The  forms  produced  by  erosion 
are  infinitely  various,  depending  upon  the  kind  of  rock 
and  upon  the  amount  and  style  o.'  folding.  They  are, 
therefore,  of  great  interest  also  as  revealing  interior  struc- 
ture. We  can  only  touch  very  lightly  on  a  few  of  the 
most  common  and  characteristic  forms. 

1.  Horizontal  Strata. — These,  when  sufficiently  hard, 
give  rise  to  table  forms,  the  top  of  the  table  being  deter- 
mined by  a  hard  stratum  of  some  kind,  as  saiidstone,  or  by 
a  lava-flow.  In  the  latter  case,  however,  we  have  this 
form,  whatever  be  the  position  of  the  underlying  strata 
(see  Fig.  6,  page  24).     Good  examples  of  this  form  are 


Fig.  151.— Table-mountains. 

seen  in  Illinois  and  in  Tennessee  (Fig.  151),  and  espe- 
cially in  the  mesas  of  the  Plateau  region  (Fig.  7,  page  25). 
If,  on  the  contrary,  the  horizonal  strata  are  soft,  and 
yield  easily  to  erosion,  they  are  worn  into  the  most  fan- 
tastic forms — conical,  castellated,  pinnacled — such  as  are 
found  in  the  ''Bad  Lands"  of  the  West,  which  are  pro- 


248 


STRUCTURAL   GEOLOGY. 


duced  by  erosion  of  the  dried-up  lake-deposits  of  this 
region  (Fig.  152). 

2.   Gently  Undulating  Strata. — These,  also,  by  ero- 
sion give  rise  to  table-topped  mountains  ;  but,  if  carefully 


Fig.  152.— Maiivaises  Terres,  Bad  Lands.    (After  Hayden.) 

examined,  the  ridges  are  seen  to  be  synclinal^  and  the  val- 
leys anticlinal.  Fine  examples  of  this  form  are  found  on 
the  western  slopes  of  the  Appalachian  chain,  where  the 
folds  of  the  strata  are  dying  away  in  gentle  undulations 
(Fig.  149).     The  reason  of  this  form  is  that  the  hollows 


Fig.  153.— Section  of  coal-field  of  Pennsylvania.     (AfUir  Lesley.) 

become  hardened  by  compression,  while  the  original 
saddles  are  loosened  or  even  broken  by  tension,  and  ero- 
sion therefore  takes  effect  mainly  on  these  latter. 


STRUCTURES  COMMON  TO  ALL  ROCKS. 


249 


3.  Hig^hly  Inclined  Outcropping  Strata. — These 
give  rise  to  sharp  ridges,  determined  each  by  the  outcrop 
of  a  hard  stratum,  with  intervening  valleys  determined  by 
the  outcrop  of  softer  strata  (Fig.  154).     This  structure  is 


Fig.  154.- 


-Parallel  ridges. 


finely  displayed  on  the  flanks  of  Western  mountains  and 
the  mountains  of  Tennessee,  and  especially  in  the  moun- 
tains of  Virginia.  Standing  on  the  top  of  Warm  Springs 
Eidge,  twelve  or  more  mountain-waves  may  be  counted, 
each  crest  determined  by  the  outcrop  of  a  hard  sandstone. 

4.  Very  Gently  Inclined  Outcropping-  Strata. — 
These,  in  the  Plateau  region,  give  rise  to  a  remarkable 
series  of  nearly  level  tables,  terminated  by  cliffs,  a  hard 
stratum  forming  the  surface  of  the  table.  In  Fig.  156, 
taken  from  Powell,  the  successive  tables  are  fifteen  to 
twenty  miles  wide,  and  the  cliffs  1,500  to  2,000  feet  high. 
The  manner  in  which  these  are  formed  is  illustrated  in 
the  diagram,  Fig.  155,  in  which  a,  b,  c,  d  are  hard  strata. 
The  dotted  space  shows  the  general  erosion. 

5.  Highly  Metamorphic  and  Granitic  Rocks. — 
These  reveal  internal  structure  much  less  perfectly  than 


Fig.  155.— Di*tted  lines  show  material  carried  away  by  erosion. 


unchanged  stratified  rocks.  Usually  the  inequalities  are 
very  irregular,  the  peaks  being  determined  by  harder,  and 
the  valleys  by  softer,  spots.  In  some  cases,  however,  the 
peculiar  forms  may  be  easily  explained.     Thus,  in  the 


250 


STRUCTURAL   GEOLOGY. 


Fiu.  156.— Bird's-eye  view  of  tlie  Terrace  Caiiou.    (After  Powell.) 


STRUCTURES   COMMON  TO  ALL  ROCKS. 


251 


high  Sierra  region,  a  remarkable  dome-structure  is  very 
characteristic  of  the  scenery.  This  is  determined  by  a 
huge  concentric  structure  of  the  granite,  as  in  Fig.  157. 


Fig.  157.— Ideal  section  showing  dome-structure.    Dotted  line  above  shows  original 

surface. 

This,  however,  must  not  be  confounded  with  arched  strata. 
These  great  domes  are  still  scaling  off  concentrically. 

6.  Outbursts  of  Igneous  Rocks  through  Dikes 
often  give  rise  to  prominent  ridges  on  account  of  their 
superior  hardness.  Examples  are  found  in  the  trap  ridges 
of  the  Connecticut  Valley  and  in  many  other  places. 

7.  The  Nature  of  the  Erosive  Agent. — The  scenic 
forms  of  mountains  are  also  largely  determined  by  the  na- 
ture of  the  erosive  agent.  Simple  water  tends,  by  erosion, 
to  form  rounded  summits  and  ridges,  and  narrow  V-shaped 
gorges.  Ice,  on  the  contrary,  tends  to  make  pinnacled 
summits  {aiguilles)  and  comb-like  ridges,  and  broad, 
meadow-like  valleys. 


CHAPTER  VI. 

DENUDATION,    OR   GENERAL  EROSION. 

Definition. — Denudation  is  a  term  used  to  designate 
the  aggregate  results  of  all  erosive  agents.  Its  correla- 
tive is  sedimentation.  In  the  preceding  pages  we  have 
given  the  effects  of  erosion  in  many  individual  cases  ;  but 
some  general  idea  of  the  amount  which  has  taken  place, 
under  the  action  of  all  agents  throughout  all  geological 
times,  and  some  very  general  estimate  of  geological  time 
based  thereon,  seem  important,  as  a  fitting  preparation 
for  Part  III,  or  Historical  Geology,  which  deals  especially 
with  time. 

Agents  of  Erosion. — The  possible  agents  of  erosion 
are — 1.  Rain  and  rivers.  2.  Snow  and  ice.  3.  Waves 
and  tides.  4.  Oceanic  currents.  Of  erosion  by  the  last, 
we  have  no  observation.  Oceanic  currents  run  on  a  bed 
and  between  banks  of  still  water,  and  therefore  produce 
no  erosion  (page  48).  We  may  probably  leave  them  out 
of  account.  Waves  and  tides  are  very  j)owerful  erosive 
agents,  but  their  action  is  confined  wholly  to  the  shore- 
line. It  has  been  estimated  that,  though  so  conspicuous, 
their  aggregate  effect  is  certainly  less  than  one  fifth  that 
of  rain  and  rivers.  Snow  is  but  a  different  form  of  rain, 
and  glaciers  a  different  form  of  rivers  ;  therefore,  in  so 
rough  an  estimate  as  we  are  about  to  make,  we  may  safely 
base  our  estimate  upon  the  action  of  rain  and  rivers. 
Our  object,  then,  will  be  to  give  some  very  general  idea 
of  the  amount  of  denudation  which  has  taken  place  in 
252 


DENUDATION,   OR  GENERAL  EROSION.        253 

geological  time  ;  then  of  the  rate  of  rain  and  river  erosion ; 
and  then  a  rough  estimate  of  the  time  necessary  to  do  the 
work. 

Modes  of  determining'  Amount  of  Denudation. — 

There  are  many  ways  in  which  geologists  determine  the 
amount  of  denudation.     In  case  of  faults,  as  in  Fig.  158, 


Pig.  158. 

in  which  the  strong  line,  a  a,  represents  actnal  surface, 
there  must  have  been  great  erosion  to  obliterate  all  sur- 
face indications  of  the  slip.  Now,  there  are  cases  of  slips 
20,000  feet  vertical,  as  in  Pennsylvania  and  on  the  north 
side  of  Uintah,  in  which  surface  indications  are  entirely 
removed  by  erosion.  Again,  in  case  of  isolated  erosion- 
peaks,  like  Fig.  159,  it  is  evident  that  the  whole  interven- 


Fig.  159.— Denudation  of  red  sandston*-,  northwest  noast  of  Eoss-shire,  Scotland. 


ing  country  has  been  carried  away.  Noa/,  such  peaks  are 
often  2,000  to  3,000  feet  high.  But  the  most  universal 
means  of  estimating  the  amount  of  erosion  is  by  resto- 
ration of  folded  strata.  This  is  shown  in  Fig.  160,  and  in 
many  of  the  preceding  figures  on  mountains. 

By  all  these  methods  it  has  been  estimated  by  British 


254 


STRUCTURAL   GEOLOGY. 


geologists  that  at  least  11,000  feet  of  thickness  has  been  re- 
moved from  the  whole  mountainous  and  hilly  portions  of 
the  British  Isles,  and  by  American  geologists  that  20,000 


Fig.  160.— Sectiou  acrosB  middle  Tennessee.    The  dotted  lines  show  the  amount  of 
multer  removed. 

feet  have  been  removed  from  the  Appalachian  region. 
This  has  all  taken  place  since  the  Palaeozoic,  which  is 
certainly  not  more  than  one  quarter  of  the  recorded  history 
of  the  earth.  But  the  finest  examples  are  from  the  Pla- 
teau region.     Fig.  161  represents  a  portion  of  the  Uintah 


Fig.  161.— Uintah  Mountains— upper  part  restored,  showing  fault ;  lower  part  shoe- 
ing the  present  condition  as  produced  by  erosion.    (After  Powell.) 

Mountains,  the  lower  portion  as  it  really  is,  the  upper  as 
it  would  be  if  restored  with  its  great  fault.  According  to 
Powell,  25,000  feet  have  been  removed  from  the  whole 
area  represented,  and  this,  too,  since  the  beginning  of 
the  Tertiary,  which  is  but  a  small  fraction  of  geological 
time.     According  to  Powell  and  Dutton,  over  the  whole 


DENUDATION,    OR  GENERAL  EROSION.        255 

Plateau  region,  an  area  of  not  less  than  200,000  square 
miles,  an  average  of  6,000  to  8,000  feet  and  an  extreme 
of  12,000  feet,  has  been  removed  by  erosion,  and  all  since 
the  Middle  Tertiary.  From  these  examples  it  is  impos- 
sible to  resist  the  conclusion  that  the  average  erosion 
over  all  land-surfaces  has  been  at  the  very  least  several 
thousand  feet. 

There  is  another  way  of  making  the  estimate  of  the 
amount  of  general  erosion.  Evidently  the  correlative  and 
measure  of  erosion  is  sedimentation.  The  debris  of  ero- 
sion have  been  accumulated  as  stratified  rock.  Now,  the 
average  thickness  of  strata  can  not  be  less  than  several 
thousand  feet.  Taking  it  only  as  2,000  feet  (it  is  certainly 
very  much  more),  since  the  area  of  ocean  is,  and 
probably  always  was,  three  times  the  area  of  the  land, 
this  would  require  at  least  6,000  feet  erosion  of  all  land- 
surfaces.  We  may  therefore  say,  with  the  utmost  confi- 
dence, that  over  all  land-surfaces  more  than  6,000  feet 
thickness  has  been  removed  by  erosion. 

Time. — Now,  we  have  seen  (page  19)  that  the  rate  of 
rain  and  river  erosion  is  about  one  foot  in  5,000  years. 
At  this  rate  it  would  take  30,000,000  years  to  do  the  work 
which  we  actually  find  has  been  done.  The  time  was 
probably  much  greater.  Exceptions  may  be  taken  to 
some  points  of  our  calculation,  but,  we  are  sure,  not  to 
the  result.  But  this,  be  it  more  or  less,  represents  only 
recorded  history.  Beyond  this,  again,  is  the  infinite  abyss 
of  the  unrecorded. 


PART  III. 
HISTOEICAL  GEOLOGY. 

CHAPTER  I. 

GENERAL   PRINCIPLES. 

Geology  is  essentially  a  history.  But  there  are  two 
points  of  view  from  which  history  may  be  studied,  viz. :  1. 
As  a  chronicle  of  thrilling  events.  2.  As  the  science  of 
the  laws  of  succession,  and  of  the  causes  of  these  events. 
The  interest  in  the  one  case  is  dramatic,  in  the  other, 
scientific.  The  one  addresses  itself  mainly  to  the  imagi- 
nation, the  other  to  the  reason.  It  is  almost  unnecessary 
to  say  that  geology  is  a  history  in  which  the  second  ele- 
ment predominates.  It  is  a  history  of  the  evolution  of 
the  earth  and  of  its  inhabitants.  Now,  there  are  certain 
general  laws  of  evolution  in  all  departments  of  nature — 
'certain  general  principles  underlying  all  history.  The 
most  important  of  these  we  wish  to  fix  in  the  mind  of 
the  pupil  by  comparing  geology  with  human  history  : 

1.  Human  history  is  divided  and  subdivided  into  eras, 
ages,  periods,  epochs,  etc.,  determined  by  great  events. 
These  divisions  of  time  are  recorded  in  separate  volumes,  ' 
chapters,  sections,  etc.,  according  to  their  importance. 
So,  also,  the  history  of  the  earth  is  divided  into  eras,  ages, 
periods,  etc.,  determined  by  great  changes  in  physical 
geography,  climate,  and  forms  of  organisms  ;  and  these 
2m 


GENERAL  PRINCIPLES.  257 

divisions  of  time  are  recorded  in  separate  rocii  systems, 
rock  series,  vock formations,  according  to  their  importance. 

2.  These  divisions  of  time,  in  human  history,  usually 
graduate,  more  or  less  insensibly,  into  each  other.  Yet, 
at  certain  points,  called  revolutio7is,  the  steps  of  change 
are  more  rapid.  So,  also,  in  geological  history,  the  eras, 
ages,  periods,  etc.,  usually  graduate  into  each  other. 
And  yet  there  are  certainly  here,  also,  times  of  revolution, 
in  which  the  steps  of  change  are  far  more  rapid.  Thus  all 
history,  human  or  geological,  consists  of  periods  of  com- 
parative quiet  and  prosperity,  during  which  the  forces  of 
change  are  gathering  strength  ;  and  periods  of  revolution, 
when  these  forces  show  themselves  in  conspicuous  effects. 

3.  In  human  history,  what  is  distinctively  called  an  age, 
is  marked  by  the  dominance  of  some  characteristic  social 
force  or  principle.  Thus,  we  have  had  an  age  of  chivalry, 
and  we  look  forward  to  an  age  of  reason.  So,  also,  in 
geology,  what  is  distinctively  called  an  age,  is  marked  by 
the  dominance  of  some  particular  class  of  animals  or 
plants.  Thus,  we  have  an  age  of  mollusks,  an  age  of 
fishes,  an  age  of  reptiles,  etc.,  in  which  these  several 
classes  are  successively  the  dominant  types.  Now,  since 
the  divisions  graduate  into  each  other,  it  is  to  be  expected 
that  the  characteristic  of  each  age  will  commence  in  the 
preceding  age.    This  we  shall  call  the  law  of  anticipation. 

4.  In  human  history  each  dominant  characteristic,  of 
course,  arises,  culminates,  and  declines ;  but  it  does  not, 
therefore,  perish.  It  only  becomes  subordinated  to  the 
next  coming  and  higher  characteristic,  and  society  thus 
becomes  not  only  higher  and  higher,  but  also  more  and 
more  complex  in  its  structure.  So,  also,  in  geology  we 
shall  find  that  as  each  dominant  class  culminates  and 
declines,  it  does  not  perish,  but  only  becomes  subordi- 
nated to  the  incoming  and  higher  dominant  class,  and 
thus  the  whole  organic  kingdom  becomes  not  only  higher 
and  higher,  but  also  more  and  more  complex  in  its  struc- 

Le  Contk,  Geol.  17 


258 


HISTORICAL   GEOLOGY. 


ture  as  a  whole.  This  is  represented  in  the  diagram  (Fig. 
162),  in  which  ^  ^  is  the  course  of  time,  and  the  rising 
and  declining  curved  lines  the  successive  culminations  of 
five  great  dominant  classes  of  animals. 


Pig.  162. — Diagram  illustrating  the  rising,  culmination,  and  decline  of  successive 
dominant  classes,  and  the  increasing  complexity  of  the  whole. 


5.  As  in  human  history,  while  the  whole  race,  or  at 
least  Christendom,  advances  together,  and  yet  there  are 
special  differences  in  rate  or  direction  of  advance  peculiar 
to  each  country  and  constituting  its  national  civilization  ; 
so  also  in  geology,  while  the  whole  earth  and  its  inhabi- 
tants in  every  part  are  affected  with  a  common  onward 
movement  in  evolution,  yet  there  are  special  differences 
in  rate  or  direction  of  evolution,  characteristic  of  each 
great  division  of  the  earth.  The  most  marked  example 
of  this  is  Australia,  which  is  far  behind  other  continents 
in  the  march  of  evolution. 

6.  In  a  written  human  history,  there  are  two  ways  in 
which  we  may  judge  of  the  subdivisions,  viz. :  1.  By  the 
artificial  divisions  of  the  record,  i.  e.,  volumes,  chapters, 
etc.;  or,  2.  By  the  nature  of  the  most  important  con- 
tents. In  a  well-written  history  these  will  correspond 
with  each  other.  So  also  in  geology  there  are  two  modes 
of  separating  and  determining  the  limits  of  the  great 
divisions  and  subdivisions  of  earth-history,  viz. :  1.  By 
unconformity  of  the  rock  record  ;  or,  2.  By  the  change 
in  the  organic  contents.  These  usually  correspond,  be- 
cause they  are  produced  by  the  same  cause.  But,  if 
there  be*  a  discordance  (as  there  may  be  locally),  then  we 
follow  the  changes  in  the  organic  forms,  rather  than  the 
unconformity  of  the  rocks. 


GENERAL  PRINCIPLES. 


259 


Divisions  of  Geological  History.     Eras. — It  is  on 

these  principles  that  the  whole  history  of  the  earth,  and 
the  rocks  in  which  it  is  recorded,  has  been  divided  and 
subdivided.     The  primary  divisions,  or  eras,  are  five  in 


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number,   each   embodied   in  a  corresponding  system  of 

rocks  :   1.  Archaiozoic,*  in  tlie  Archaean  or  Primary  or 

Laurentian  system  of  rocks  ;  3.   Paleozoic,!  in  the  Palaeo- 

*  Archaeozoic  =  primeval  animal  life.      \  Paleozoic  =  old  life. 


260  HISTORICAL   GEOLOGY. 

zoic,  or  sometimes  called  transition  system  of  rocks ; 
3.  Mesozoic^*  in  the  Secondary  system  ;  4.  Cenozoic^f  in 
the  Tertiary  and  Quaternary  systems  ;  and,  5.  Psycho- 
zoic,  J;  in  the  present  system  of  sediments.  These  five  are 
separated  in  the  diagram  (Fig.  163)  by  the  heavy  lines, 
and  their  names  are  given  on  the  right. 

How  separated. — These  primary  divisions  (unless  we 
except  the  last)  are  separated  by  a  universal  or  almost 
universal  unconformity,  indicating  wide-spread  changes 
in  physical  geography  at  these  times ;  and  by  sweeping 
changes  in  organic  forms,  involving  not  only  species,  but 
genera,  families,  and  orders.  The  changes  between,  the 
last  two  were  not  so  great  either  in  the  rocks  or  in  the 
organisms,  but  the  introduction  of  man,  and  the  sweep- 
ing changes  going  on  now  by  his  agency,  are  deemed  of 
sufficient  importance  to  make  this  a  primary  division. 

Ages. — The  whole  history  of  the  earth  is  divided,  on 
a  different  principle,  into  seven  ages,  characterized  each 
by  a  dominant  class.  In  some  cases  these  correspond  to, 
and  in  some  are  subdivisions  of,  the  eras.  These  ages, 
and  their  corresponding  rocks,  when  they  are  subdivisions, 
are  separated  usually  by  unconformity,  but  not  so  uni- 
versal ;  and  by  changes  of  organisms,  but  not  so  sweeping. 
They  are — 1.  Archaean  or  Eozoic  age,  corresponding  to 
the  Eozoic  era.  2.  Age  of  Mollusks,  or  age  of  Inverte- 
brates, corresponding  to   the   Silurian    system  of  rocks. 

3.  The  age  of  Fishes,   corresponding  to  the  Devonian. 

4.  The  age  of  Acrogens,  or  age  of  Amphibians,  corre- 
sponding to  carboniferous  strata.  5.  The  age  of  Reptiles, 
corresponding  to  the  Mesozoic  era  and  Secondary  rocks. 
6.  The  age  of  Mammals,  corresponding  to  the  Cenozoic 
era  and  the  Tertiary  and  Quaternary  rocks.  7.  The  age 
of  Man,  corresponding  to  the  Psychozoic  era  and  the 
present  sediments. 

*  Mesozoic  =  middle  Ufa.     f  Cenozoic  =  recent  Hfe,     X  Psychozoic  = 
rational  life. 


A 


GENERAL  PRINCIPLES. 


261 


Tapir,  Peocarj,  BL'ion,  Llama. 
£quus.    Megatherium,  Mylodim,  Etepha* 


Plioliippus  13eds, 

PHohippus,  Mastodon,  Bob,  etc. 


Miohippus  Beds. 

Miohippua,  Diceratheriv/m,  Thinohyns. 

Oreodon  Beds. 

Edentates,  Hyamodon,  Hyraeodon. 

Brontotheriuin  Beds. 

Meeohippus,  Menodus,  Elothwium. 


Diplacodon  Beds. 

^nhippus,  Amynodon. 

Dinoceras  Beds. 

Tinoceraa,  Uintathenum,  Limnohyvs, 
Orohippui,  Helaletes,  CoUmoceras 

Corj^pFiodon  Beds. 

Eohippua,  Monkeys,  Carnivores,  Ungulates,  Tillotlonts,  Rodents,  Serpents. 


Lignite  Series. 

Hadroaaurua,  Ih-yptoaaurua. 


Pteranodon  Beds. 

Birds  with  Teeth,  Heeperomia,  lehthyomia. 
Pterodactyls,  Plesiosaurs. 


Dakotah  Group. 
Comanche  Group. 


Atlantosaurus  Beds. 

Sinoeaurg,  Apatoaawrua,  AUoaaurua,  Na/noaaxtrua. 


Turtlea.    iMpioaourus. 


Connecticut  River  Beds. 


Dinosaur  Foot-prints,  Amphiaaurua. 
Crocodiles  (Belodon). 


Permian. 

First  Reptiles. 


Coal-Measures. 


Sub-carboniferous. 

First  knovn  AmphiUans  (Labyrinthodflntg). 


Corniferous. 


Schoharie  Grit. 

First  Fish  Fauna. 


Upper  SiUirian. 

First  known  Fishes. 

Lower  Silurian. 


Primordial. 


Huronian. 


Lauren  tian. 


No  Vertebrates  known. 


Fig.  164. — Section  of  the  earth's  crust,  to  illustrate  vertebrate  lite  in  America. 
^Sliehtlv  modified  from  Marsh.) 


262  HISTORICAL  OKOLOOY. 

In  the  diagram  (Fig.  163)  the  different  rock-systems 
are  placed  one  on  top  of  the  other,  and  the  vertical  black 
spaces  represent  by  their  breadth  the  relative  dominance 
of  different  classes  at  different  times. 

Periods  and  Epochs. — The  subdivisions  of  these  again 
into  periods  and  epochs  are  founded  on  more  local  uncon- 
formities, and  especially  on  less  important  changes  in  tho 
species. 

We  have  already,  on  page  204,  given  a  schedule  of  the 
most  important  divisions  and  subdivisions  adopted  in  this 
work  ;  but  we  shall  not  treat  separately  all  of  these.  As 
in  human  history,  so  in  geology,  the  earliest  times  are  little 
known,  and  are  touched  lightly.  As  we  come  toward  the 
present,  and  events  thicken,  we  shall  take  up  subdivisions 
more  and  more — first  ages,  then  periods,  and,  finally,  even 
epochs.  We  give  here  also  (Fig.  164)  a  generalized  sec- 
tion of  American  strata,  which  will  be  found  useful  for 
reference.  It  must  not  be  supposed,  however,  that  all 
these  strata  occur  in  any  one  place.  It  is  an  ideal  section, 
in  which  all  the  most  important  American  strata  occurring 
in  different  places  are  brought  together  and  arranged  in 
the  order  of  time. 

We  are  now  ready  to  commence  a  rapid  survey  of  the 
history  of  the  earth.  But  it  must  be  understood  that  we 
can  commence  only  where  the  record  commences.  Before 
this  is  the  abyss  of  the  unrecorded,  of  which  we  know 
nothing  positive.  Before  the  historic  is  the  prehistoric  ; 
no  history  can  recall  its  own  beginning. 


CHAPTER  11. 

ARCH^AN   SYSTEM   AND   ARCHEOZOIC   BRA. 

The  events  recorded  in  this  oldest  system  of  rocks,  in 
this  first  volume  of  the  book  of  time,  are  so  few  and  so 
imperfectly  recorded  that  their  chief  interest  consists  in 
the  fact  that  they  are  the  first.  There  is  a  fascination 
about  the  beginning — the  mythical  period — of  all  history. 
The  distinctness  of  this  system  was  for  a  long  time  un- 
recognized. It  has  now,  chiefly  by  the  labors  of  Ameri- 
can geologists,  been  completely  established.  In  no  single 
instance  have  these  rocks  been  found  to  graduate  into  the 
Paleozoic.  There  is  absolutely  everywhere  an  uncon- 
formity between  them  and  every  other  system.  No  such 
complete  and  universal  break  occurs  anywhere  else  in  the 
rocky  series  as  occurs  here  (Fig.  165).     It  is,  therefore. 


1  2 

Fig.  165.— Section  showing  Primordial  unconformable  on  the  Archaean  :  1,  Archaean 
or  Laurentian  ;  2,  Primordial  or  lowest  Silurian.    (After  Logan.) 

properly  called  a  distinct  system  and  a  distinct  era — 
more  distinct,  in  fact,  than  any  other. 

Here,  then,  we  have  the  oldest  knoivn  rocks.  Are  they, 
then,  absolutely  the  oldest — the  primitive  rocks,  as  some 
imagine  ?  By  no  means.  They  are  stratified  rocks,  and 
therefore  consolidated  sediments,  and  therefore,  also,  the 

263 


264 


HISTORICAL   OEOLOOY. 


dShris  of  still  older  rocks,  of  which  we  know  nothing. 
Thus,  we  seek  in  vain  for  the  absolutely  oldest,  the  primi- 
tive crust.  As  already  said,  no  history  can  write  its  own 
beginning. 

Character  of  these  Rocks. — We  can  only  say,  in 
brief,  that  they  do  not  differ  very  conspicuously  from 
metamorphic  rocks  of  other  times.  They  were  probably 
originally  sands,  clays,  and  limestones,  much  like  those  of 
other  times  ;  but,  in  this  case,  alivays  very  highly  meta- 
morphic and  strongly  crumpled  (Fig.  166).  The  sands  are 
thereby  changed  into  quartzites,  the  clays  into  schists. 


Fig.  166,— Contortion  of  Laurentian  Btrata.    (After  Logan.) 


gneisses,  and  even  granites,  and  the  limestones  into  mar- 
bles. Along  with  these,  however,  are  associated  two  kinds 
of  beds,  which  are  worthy  of  note,  viz.,  beds  of  iron-ore 
and  beds  of  graphite.  In  Canada  the  whole  series  is  not 
less  than  40,000  feet  thick. 

The  greatest  beds  of  iron-ore  known  in  any  strata  are 
found  here.  The  great  iron-ore  beds  of  Sweden,  of  Lake 
Superior  (Fig.  167),  of  New  Jersey,  and  the  Iron  Moun- 
tain of  Missouri,  are  in  these  rocks. 
Recently,  in  southern  Utah,  in  rock 
of  this  age  (or  possibly  later),  have 
been  found  the  greatest  iron-de- 
posits, perhaps,  in  the  world.  The 
strata  here  stand  on  edge,  and  the 
beds  of  iron-ore,  being  very  hard,  have  been  left  by 
erosion  standing  out  as  black,  castellated,  inaccessible 
crags,  300  feet  high,  1,000  feet  long,  and  500  feet  thick. 
In  Canada  and  elsewhere  graphite  also  occurs  in  immense 
beds,  sometimes  pure,  sometimes  mixed  with  the  rock. 


Fig.  167. 


ARCH^AN  SYSTEM  AND  ARCHEOZOIC  ERA.     265 

Area. — 1.  These  rocks  cover  the  whole  of  Labrador, 
nearly  the  whole  of  Canada  (passing  into  New  York  in 
^he  region  of  the  Adirondacks),  then  extend  northwest 
probably  to  the  Arctic  regions.  This,  the  greatest  Ar- 
chaean area  in  North  America,  forms  a  broad,  open  V, 
inclosing  in  its  arms  Hudson  Bay.  2.  The  next  largest 
area  is  a  broad  space  extending  from  New  England  to 
Georgia,  including  the  Blue  Ridge  and  the  eastern  slope 
of  the  Appalachian.  3.  The  axes  of  many  of  the  great 
mountain-ranges,  such  as  the  Colorado,  Park,  and  Wah- 
satch  Ranges,  and  possibly  the  Sierra  Nevada.  4.  Some 
small,  isolated  spots,  one  in  Texas  and  one  in  Missouri. 
In  the  map  (page  272)  these  are  represented  by  v  • 

Physical  Geography. — These  being  stratified  rocks, 
it  is  evident  that  the  whole  Archaean  area  was  sea-bottom 
at  that  time.  Where,  then,  was  the  land  from  which 
this  debris  was  derived  ?  Of  this  we  know  nothing. 
Some  have  thought  that  it  was  to  the  northeastward. 
We  shall  see  hereafter  that  the  continent  developed 
southward  and  westward. 

Amount  of  Time. — The  Archaean  rocks  are  of  enor- 
mous thickness,  probably  equal  to  all  other  subsequent 
rocks  put  together.  The  amount  of  time  represented  is, 
therefore,  probably  equal  to  all  the  rest  of  recorded  his- 
tory of  the  earth.  And  yet  how  meager  the  record  !  It 
is  the  same  with  the  earliest  human  history. 

Life. — Did  any  living  thing  exist  at  that  time  ?  This 
is  a  very  important  question,  but  we  can  not  yet  answer 
it  with  absolute  certainty.  There  are,  however,  some 
good  evidences  of  life  :  1.  Iron-ore  is  accumulated  now, 
and  therefore  probably  also  in  earlier  times,  only  by  means 
of  decaying  organic  matter  (page  89),  and  is,  therefore, 
justly  regarded  as  a  sign  of  life  and  a  measure  of  its 
quantity.  2.  Graphite  is  regarded  as  the  highest  anthra- 
citic  condition  of  coal;  and  coal  is  a  positive  sign  of 
organic  matter,  and  therefore  of  the  previous  existence 


266  HISTORICAL  GEOLOGY. 

of  life.  Limestone,  as  we  have  seen  (pages  114-117),  Ib 
noWy  and  at  previous  geological  times,  usually,  though  not 
always,  of  organic  origin. 

I  Judging  from  these  signs,  it  would  seem  that  life  was 
not  only  present,  but  in  large  quantity.  Can  we  say  any- 
thing as  to  its  kind  ?  Are  there  any  fossils  ?  Here  we 
must  answer  still  more  doubtfully.  Some  curious  forms 
are  found  which  are  supposed  to  be  those  of  the  lowest 
order  of  animals  {compound  Protozoa).  These  have  been 
called  eozoon  or  dawn-animal,  and  therefore  some  have 
called  this  first  era  eozoic.  Most,  however,  do  not  accept 
this  animal,  and  prefer  the  name  Azoic  (no  animal  life), 
or,  better  still,  Archaean  or  Archaeozoic,  as  carrying  no 
implication. 

In  conclusion,  we  may  say  that  the  existence  of  the 
lowest  forms  of  vegetable  life  is  almost  certain,  and  of 
the  lowest  forms  of  animal  life  probable. 


CHAPTER  III. 

PALEOZOIC    ROCKS   AND   ERA. 

Section  I. — General  Description. 

The  Lost  Interval. — Between  the  Archaean  and  Pale- 
ozoic rocks  occurs  the  greatest  and  most  universal  break 
in  the  whole  stratified  series.  At  this  point  in  time 
occurred  the  greatest  and  most  wide-spread  changes  in 
physical  geography  and  climate  which  has  ever  occurred 
in  the  history  of  the  earth.  The  justification  for  this 
statement  is  found  in  the  fact  that  everywhere,  even  in 
the  most  distant  localities,  we  find  the  lowermost  Paleo- 
zoic (Primordial)  lying  unconformably  on  the  Archaean. 
No  one  has  yet  seen  the  two  series  continuous.  Now, 
when  we  remember  that  unconformity  always  means  a 
previous  eroded  land-surface  (page  192),  and  stratified 
rock  a  sea-bottom,  we  easily  perceive  how  wide-spread  the 
changes  of  physical  geography  must  have  been  at  this 
time.  Again,  when  we  remember  that  unconformity  also 
always  means  a  lost  interval  unrecorded  at  the  place  ob- 
served, and  that  the  unconformity  exists  at  all  observed 
places,  we  at  once  see  that  right  here  is  an  unrecovered, 
probably  an  irrecoverable,  lost  interval  of  time.  During 
the  lost  interval  wide  areas  of  land  existed,  which  were 
afterward  submerged  and  covered  with  Paleozoic  sedi- 
ments. As  compared  with  the  early  Paleozoic,  it  was 
evidently  a  continental  period. 

Corresponding  with  the  great  physical  changes  here, 
there  was  also  immense  advance  in  life-forms.     During 

267 


268  HISTORICAL   GEOLOGY. 

the  Archseozoic,  as  we  have  already  seen,  the  life,  if  any, 
was  only  of  the  lowest  possible  kind.  Life-forms  had 
not  differentiated  into  distinct,  recognizable  species. 
There  was  not  yet  what  could  justly  be  called  a  fauna  and 
flora.  Then  came  the  lost  interval,  represented  by  the 
unconformity.  Of  what  took  place  then  we  know  nothing. 
When  the  record  opens  again  with  the  Paleozoic,  we  have 
already  an  abundant  and  diversified  fauna  and  flora. 
};ven  in  the  lowest  Primordial  we  find  all  the  great  de- 
pi^rtments  of  Invertebrates,  and  nearly  all  the  classes  of 
these  departments,  already  represented.  It  certainly 
looks  like  a  sudden  appearance  of  somewhat  highly  organ- 
ized animals,  without  progenitors.  But  we  must  not 
forget  the  lost  interval.  It  is  probable  that  during  this 
period  of  rapid  physical  changes  there  were  also  rapid 
changes  in  organic  forms. 

It  is  for  these  reasons  that  the  Paleozoic  is  regarded 
as  opening  a  new  era,  and,  in  fact,  the  most  distinct  in 
the  history  of  the  earth.  We  have  explained  its  distinct- 
ness from  the  Archaean  below,  but  we  shall  find  hereafter 
that  it  is  almost  equally  distinct  from  the  Mesozoic  above. 
It  is  separated  on  both  sides  by  unconformity  and  by 
changes  in  life — a  distinct  volume  with,  as  it  were,  blank 
boards  on  either  side. 

Rock-System. — There  is  nothing  very  noteworthy  in 
the  character  of  the  rocks  of  the  Paleozoic.  Only  this 
may  be  said  :  as  compared  with  the  Archaean  rocks,  they 
are  far  less  universally  thick,  metamorphic,  and  crumpled. 
In  mountain-regions,  indeed,  they  are  very  thick  (40,000 
feet  in  the  Appalachian),  very  metamorphic,  and  very 
much  folded  ;  but  in  level  regions  they  are  often  much 
thinner,  entirely  unchanged,  and  level-lying.  For  exam- 
ple, in  passing  from  the  Appalachian  westward,  we  find 
the  following  four  kinds  of  change  :  1.  In  the  Appalachian 
tlie  Paleozoics  are  40,000  feet  thick  ;  they  thin  out  west- 
ward, until  at  the  Mississippi  River  they  are  only  4,000 


PALEOZOIC  ROCKS  AND  ERA.  269 

beto  2o  In  the  former,  sands,  grits,  and  clays  predomi- 
nate ;  in  the  hitter,  limestones.  3.  In  the  former  the 
rocks  are  strongly  folded ;  these  folds  die  out  through 
gentle  undulations  to  level-lying  strata  in  the  latter.  4. 
In  the  former  the  rocks  are  highly  metamorphic  ;  in  the 
latter  they  are  wholly  unchanged. 

Area  in  the  United  States, — lo  Eastern  Paleozoic 
BastUo  The  Paleozoic  rocks  cover  a  large  continuous 
area  in  the  very  best  part  of  the  United  States.  This  area 
is  bounded  on  the  north  by  the  chain  of  the  Great  Lakes  ; 
on  the  east  by  the  Blue  Ridge  of  .the  Appalachian  chain ; 
on  the  south  by  a  line  running  through  mid-Alabama, 
turning  northward  to  the  mouth  of  the  Ohio  River  ;  then 
south  through  mid-Arkansas  and  Indian  Territory  ;  on 
the  west  by  the  Western  grassy  plains.  2.  Besides  this 
great  area,  there  are  several  considerable  areas  scattered 
about  in  the  Plateau  region  and  exposures  along  flanks 
of  mountains  of  the  Plateau  and  Basin  regions. 

Physical  Geography. — The  physical  geography  of 
the  eastern  portions  of  the  North  American  Continent  in 
Paleozoic  times  can  be  made  out  with  considerable  cer- 
tainty. In  fact,  we  can  in  many  places  trace  the  Primor- 
dial shore-line.  Immediately  in  contact  with  the  Canadian 
Archaean  on  the  north,  and  the  Blue  Ridge  Archaean  on 
the  east,  are  found  patches,  or  continuous  lines  of  a  coarse 
sandstone,  which  contain  all  the  marks  characteristic  of 
shore-lines,  such  as  worm-tubes,  worm-trails,  crustacean 
tracks,  ripple-marks,  rain-prints,  etc.  This  is  the  old 
Primordial  beach.  At  the  beginning  of  Paleozoic  times, 
therefore,  the  whole  Paleozoic  basin  was  covered  by  a  sea 
which  beat  against  a  land-mass  to  the  north  (Canadian 
Archaean  area),  and  a  land-mass  to  the  east  (Blue  Ridge 
Archaean  area).  This  is  called  the  great  interior  Paleo- 
zoic Sea,  There  was  also  a  large  land-mass  in  the  Basin 
region,  and  smaller  masses,  probably  islands,  in  the  Colo- 
rado mountain-region,  but  the  exact  limits  of  these  are 


270 


HISTORICAL    GEO  LOU  Y. 


not  known.  The  map  (Fig.  108)  represents  the  present 
state  of  our  knowledge  on  this  subject.  It  is  probable, 
however,  that  the  Eastern  land-mass  (Blue  Ridge  Archaean 
area)  was  larger  than  represented,  having  been  subse- 
quently covered  by  later  deposits,  and  partly,  even  now, 
by  the  Atlantic  Ocean. 

The  change  in  the  rocks,  in  passing  westward  from  the 
Appalachian  region,  is  completely  explained  by  the  posi- 
tion of  the  Appalachian  region  and  the  subsequent  forma- 


FiG.  168.— Map  of  physical  geography  of  Primordial  times  :  existing  seas  and  lakes, 
black  ;  continental  seas  of  that  time,  light  shade  ;  land  of  that  time,  white.  The 
white  dotted  line  shows  the  probable  shore-line  of  2  at  this  time. 


tion  of  the  mountains.  This  region  was  then  the  marginal 
bottom  of  the  interior  sea,  receiving  abundant  and  coarse 
sediments,  whicli  became  finer  and  thinner  seaward. 
This  thick  marginal  line  then  yielded,  was  strongly  folded 
and  highly  metamorphosed  in  the  act  of  mountain- 
making  which  took  place  at  the  end  of  the  Paleozoic. 


PALEOZOIO  ROCKS  AND  ERA.  271 

Growth  of  the  Continent  during  Paleozoic  Times. 

— The  map  (Fig.  168)  represents  the  continent  at  the 
heginning  of  the  Paleozoic.  But  d^i^ing  that  era  there 
was  a  steady  growth  from  this  nucleus  by  addition  south- 
ward and  westward,  until,  at  the  end,  the  whole  of  the 
Paleozoic  areas  were  reclaimed  from  the  sea,  and  the 
continent  was  nearly,  though  not  exactly,  that  represented 
on  page  349.  It  will  be  seen  that  the  continent  was 
already  outlined  at  the  beginning  of  the  era,  and  was 
steadily  developed  toward  its  present  forme  "We  shall 
hereafter  trace  this  development  to  its  completion. 

Subdivisions  of  the  Paleozoic. — The  Paleozoic  era 
and  strata  are  divided  into  three  ages,  each  represented  by 
corresponding  rock-systems  :  1.  The  age  of  MoUusks,  or  of 
Invertebrates,  represented  by  the  Oamhrian  and  Silurian 
system  ;  2.  The  age  of  Fishes,  by  the  Devonian  ;  3.  The 
age  of  Acrogen  Plants  and  Amphihian  Animals,  by  the 
Carboniferous,  These  three  rock-systems,  in  many  parts 
of  the  world,  are  unconformable  with  each  other  ;  but  in 
the  United  States  they  are  usually  entirely  conformable. 
Nevertheless,  their  life-systems  (organic  forms)  are  here, 
as  everywhere,  quite  different. 

All  these  subdivisions  are  well  represented  in  the  Pa- 
leozoic basin  of  the  United  States  (Fig.  169).  In  the  fol- 
lowing map  of  the  main  divisions  of  the  geological  strata 
of  the  Eastern  United  States,  the  rocks  representing  these 
three  ages  are  all  shown.  It  is  important  to  study  this 
map  well,  for  it  will  be  referred  to  frequently  hereafter  Id 
connection  with  more  recent  strata. 

Section  IL — Lowee    Paleozoic   or  Cambrian    and 
Silurian  System.    Age  of  InvertebrateSo 

Bocks  ;  Name. — ^These  rocks  are  called  Cambrian  and 
Silurian,  from  the  Roman  name  for  Wales  and  the  Welsh, 
because  they  were  first  studied  in  Wales,  by  Sedgwick  and 


372 


HISTORICAL   GEOLOGY, 


i^^ 

i 

PALEOZOIC  ROCKS  AND  ERA.  373 

Murchison.  But  they  are  far  more  perfectly  represented 
in  the  United  States. 

Area. — It  will  be  seen,  by  reference  to  the  map.  Fig. 
169,  that  in  the  g"8at  Paleozoic  basin  these  rocks  form 
an  irregular  border  to  the  Canadian  and  Blue  Eidge 
Archaean  areas.  These  borders  were  marginal  sea-bot- 
toms at  the  beginning  of  the  Silurian  times,  and  were 
elevated  and  reclaimed  during  and  at  the  end  of  that 
time.  There  are  many  other  smaller  areas  in  the  West, 
but  these  can  not  be  defined. 

Physical  Geography. — We  have  already  given  this 
for  the  beginning  of  the  age  in  the  map.  Fig.  168.  For 
the  end  of  the  age,  as  just  stated,  we  must  add  the  Silu- 
rian area  to  the  Archaean  area.  There  was  also  at  the 
end  added  a  large  island  of  Silurian  sea-bottom  in  Ohio 
and  Tennessee  (see  map.  Fig.  169). 

Subdivisions. — The  Lower  Paleozoic  rocks  are  sub- 
divided into — 1.  Primordial,  or  Cambrian  ,  2.  Lower 
Silurian ;  3.  Upper  Silurian  ;  and  these,  again  subdivided, 
as  shown  in  the  following  schedule.  We  simply  give  these 
by  name  for  reference,  if  necessary,  but  will  treat  of  the 
whole  Cambrian  and  Silurian  together  : 

f  Helderberg  period 

8.  Upper  Silurian.     \  Sahna 

1  Niagara  ** 

2.  Lower  Silurian.  \  ^^^^^^^^ 

^  (  Canada  *"* 

1.  Cambrian,      or  j 

Primordial.  {Primordial       *' 

Life-System. 

We  have  already  spoken  of  the  apparent  suddenness  of 
the  appearance  of  a  somewhat  diversified  fauna  in  the 
Primordial,  and  accounted  for  it  by  the  existence  of  a  lost 
interval.  Immediately  after  the  Primordial  the  fullness 
of  Paleozoic  life  became  really  wonderful.     These  early 

Lb  Conte.  Gbol.  18 


274 


HISTORICAL   GEOLOGY, 


seas  seem  to  have  swarmed  with  a  life  as  abundant  as  any 
now  existing,  but  wholly  different  in  species,  in  genera, 
and  even  in  families,  not  only  from  any  710 w  living,  but 
from  those  living  in  any  other  geological  period.  About 
20,000  species  are  described  from  the  Paleozoic,  and  of 
these  at  last  one  half,  i.  e.,  10,000  species,  are  from  the 
Silurian  ;  and  of  course  these  are  but  a  very  small  frac- 
tion of  the  number  which  actually  existed.  The  number 
being  so  great,  and  the  forms  so  unfamiliar  to  the  pupil, 
it  is  impossible  to  do  more  than  mention  and  figure  a  few 
of  the  most  common  and  striking  forms. 

Plants. 

The  only  kind  of  plants  which  are  found  so  early  are 
allied  to  sea-weeds.*  As  it  is  very  difficult  to  determine 
these  species  from  the  very  imperfect  impressions  of  them 
left  in  the  rocks,  we  shall  call  them  by  the  general  name 


Fig.  170. 


Fig.  in. 


Pigs.  170,  171.— Silurian  plants  :  170.   Sphenothallus  angustifolius.    171.  Buthotre- 
phis  gracilis. 


of  Fucoids,  i.  e.,  /wcws-like  plants,  from  their  general 
resemblance  to  Fucus  (tangle  or  kelp).     We  give  a  few 

*  A  few  small  vascular  cryptogams,  allied  to  club-mosses,    have 
been  reeentlv  found  in  the  Silurian. 


PALEOZOIC  ROCKS  AND  ERA,  5^75 

(Figs.  170,  171),  to  show  their  general  appearance.     They 
belong  to  the  lowest  order  of  plants. 

Animals. 

These  are  far  more  numerous  and  diversified  than  the 
plants.  We  can  mention  only  such  as  may  be  recognized 
even  by  the  untrained  eye. 

Corals. — These  are  very  abundant,  and  seem  some- 
times to  have  formed  veritable  reefs.  There  are  three 
very  characteristic  forms,  viz.,  6'w^-corals  {Cyatliophyl- 
loids,  Figs.  172,  173),  Honey comh-cor^lB  {Favositids,  Fig. 
174),  and  C%«m-corals   {Halysitids,  Fig.   175).     These 


Fig.  173. 


Fig.  17 

Figs.  172, 173.— Cyathophylloid  corals  :  172.  Lonsdaleia  floriformis.    (After  Nichol- 
son.)    173.  Strombodes  pentagouus.    (After  Hall.) 

are  all  characteristic  of  the  Paleozoic,  and  the  last  char- 
acteristic of  the  Silurian.  Now,  any  one  can  recognize 
these,  especially  the  Honeycomh  and  Chain  corals,  and 
therefore  when  these  are  found  any  one  may  identify 
Paleozoic  or  even  Silurian  rocks. 

Hydrozoa. — In  still,  sheltered  bays,  with  fine  mud- 
bottom,  are  now  found,  attached  to  sticks,  logs,  or  shells, 
fine,  feathery  things,  which  look  like  finely  dissected  seti- 
weed  or  sea  moss.     They  are,  indeed,  gathered  by  ama- 


276 


HISTORICAL  OEOLOQY, 


teur  collectors  and  pressed  as  sea- weeds.     If  they  be  ex- 
amined with  a  lens,  they  are  seen  to  be  composed  of 


Fig.  174.  Fig.  175. 

Pigs.  174, 175.— Favositid  and  halysitid  corals  :  174.  Columuaria  alveolata.    (After 
Hall.)    175.  Halysiteecatcnulata     (After  Hall.) 

hollow,  branching  stems,  set  on  one  or  both  sides  with 
hollow  cups,  each  containing  an  animal  which,  if  kept 
undisturbed  in  sea-water,  quickly  spreads  its  thread-like 
tentacles.  These  are  the  Hydrozoa  of  the  present  day 
(Figs.  176, 177, 178).     Now,  in  fine  Silurian  shales,  which 


Fig.  176-  Fig,  177.  Pig.  178. 

Figs.  176-178.— Living  hydrozoa  :  176  and  177.  Sertularia.    178.  Plumularia. 

were  once  fine  mud,  are  found  impressions  of  animals 
probably  similar  to  these.  They  are  called  Graptolites, 
Whatever  they  be,  they  are  easily  recognized  and  wholly 


PALEOZOIC  ROCKS  AND  ERA. 


277 


characteristic  of  Silurian,  and  any  one  may  identify  Silu- 
rian by  means  of  them  (Figs.  179,  180). 


Fig.  179.  Fig.  180. 

Figs.  179,  180. —Silurian  hydrozoa  :   179.  Diplograptus  priatis.     (Alter  Nicholson.) 
180.  Graptolites  Clintonensis.    (After  Hall.) 

Echinoderms ;  Crinoids. — At  the  present  time,  if  we 
leave  out  sea-cucumbers  (Holo- 
thurians),  because,  having  no 
shells,  they  are  not  preserved  as 
fossils,  Echinoderms  are  of  three 
orders  :  1.  Echinoids,  or  sea-ur- 
chins ;  2.  Asteroids,  or  star-fishes  ; 
and,  3.  Crinoids.  The  first  two 
are  /ree-moving,  the  last  is 
stemmed.  The  first  two  are  now 
very  abundant,  the  last  r^re. 
But  in  Silurian  times  it  was  the 
reverse.  The  Echinoids  did  not 
exist  at  all,  the  Asteroids  were 
rare,  but  the  Crinoids  extremely 
abundant,  though,   of   course,   of  Fig.    181.  -  Living    crinoid. 

^  1     n        T  «•  Pentacrinus  caput-medusa!. 

species  and  genera  wholly  ditter- 

ent  from  any  now  existing  (Fig.  181).     It  is  well  to  ob- 
serve that  the  crinoid  is  a  lower  form  than  the  other  two, 


278  HISTORICAL  GEOLOOY. 

as  is  shown  by  the  fact  that  some  free  echinoderms  have 
stems  in  the  early  stages  of  life,  and  afterward  throw 
them  off  and  become  free. 

Description   of  a  Crinoid. — A  crinoid  has  a  pear- 
shaped  body,  containing  the  viscera,  set  upon  a  jointed 


Fui.  18-2.  Fig.  183.  Fig.  184. 

Figs.  182-184.— Silurian  crinoids  :  182.  Heterocrinus  simplex.    (After  Meek.)    188. 
Pleurocystitis  squamosus.    184.  Lepadocrinus  Gebhardii. 

stem,  with  mouth  on  the  top  of  the  pear,  sometimes  sur- 
rounded by  many  plumose  arms  (Fig.  182),  sometimes 
•with  few  simple  arms  (Fig.  183),  sometimes  with  no  arms 
at  all  (Fig.  184). 

Range  in  Time. — We  have  said  that  stemmed  echino- 
derms or  crinoids  continue  from  earliest  times  until  noiu 
(though  the  species  and  genera  change  repeatedly),  but 
in  diminisMng  numbers.  The  free  echinoderms,  on  the 
contrary,  have  been  constantly  increasing.  If,  then,  A  B 
(Fig.  185)  represent  the  course  of  geological  time,  and 
the  parallelogram  the  equal  abundance  of  echinoderms 
throughout,  then  the  shaded  portion  below  the  diagonal 
would,  in  a  general  way,  represent  the  constantly  decreas- 
ing stemmed f  and  the  unshaded  space  above  the  diagonal 


PALEOZOIC  ROCKS  AND  ERA, 


279 


the  constantly  increasing  free  forms.     But  are  there  any 
characters  by  which  we  may  easily  recognize  those  pecu- 


^ALEOZOIC »^ 

SILURIA N.  DEVON^    CAPBONJF^ 


STEMMED 
Fig.  185.— Diagram  showing  distribution  in  time  of  crinoids. 

liar  to  the  Silurian  ?  There  are.  Crinoids  are  subdi- 
vided into  three  main  groups,  viz.  :  1.  Crinids,  or 
plumose-armed  crinoids  (Fig.  182) ;  2.  Blastids  (Fig. 
242,  page  316),  or  bud-crinoids ;  3.  Cystids  (Figs.  183, 
184),  or  bladder-crinoids.  The  crinids  are  not  character- 
istic of  Silurian,  nor  even  of  Paleozoic ;  the  blastids  are 
characteristic  of  Paleozoic,  though  not  of  Silurian  ;  the 
cystids  are  characteristic  of  Silurian  alone.  This  is  rep- 
resented by  subdivisions  of  the  shaded  space  in  Fig.  185, 
in  relation  with  the  subdivision  of  the  Paleozoic. 

Mollusks ;  Brachiopods. — Bivalve  shells  are  divided 
into  two  great  groups,  viz.  :  1.  Common  bivalves  {Lamel- 
lihranchs) ;  and,  2.  Lamp-shells,  or  Brachiopods.  At 
present,  the  former  are  extremely  abundant,  and  the  lat- 
ter rare.  The  reverse  was  true  in  Silurian  times.  The 
distribution  in  time  of  the  two  kinds  may  be  roughly 


Fig.  186.— Diagram  showing  the  general  distribution,  in  time,  of  brachiopods  and 
lamellibranchs. 


represented  by  the  diagram  (Fig.  186).  Now,  brachio- 
pods are  very  different  from,  and  much  lower  than,  ordi- 
nary bivalves.  Lamellibranchs  have  a  right  and  left 
valve — right  and  left   gills,    etc.  ;    in    brachiopods  the 


280 


HISTORICAL  GEOLOGY, 


valves  aro  upper  and  lower,  or  a  back-piece  and  a  breast- 
plate. The  deeper  and  more  projecting  valve  is  the  ven- 
tral. From  the  point  of  this  valve  comes  out  a  fleshy 
cord,  by  which  it  is  attached.  It  is  this  which  gives  it 
the  name  of  lamp-shells,  on  account 
of  its  resemblance  to  the  ancient  lamp 
(Fig.  187).  A  large  portion  of  the  in- 
terior of  the  shell  is  occupied  by  long, 
spiral,  fringed  arms-  It  is  these  which 
gi  V  e  the  name  of  brachiopod  (arm-feet), 
although  they  are  really  gills.  These  are  attached  to 
complex,  and  sometimes  spiral,  bony  pieces.  Fig.  188  is 
a  living  brachiopod,  showing  structure.     These   shells 


Fig.  187.— Living  bra- 
chiopod.   Side  view. 


Fi».  188.— A  living  brachiopod    Terebratula  flavescene. 


are  so  extremely  abundant  in  Paleozoic,  especially  Silu- 
rian rocks,  that  these  rocks  may  often  be  identified  by 
them.  In  Figs.  189,  190,  we  give 
two  of  the  most  common  forms. 
Are  there  any  characters  by  which 
Silurian  brachiopods  can  be  easily 
distinguished  ?  Not  by  the  un- 
trained eye.  Yet  the  sqaare  shoul- 
dered forms,  like  those  figured  here, 
are  very  characteristic  of  Paleozoic,  though  not  of  Silu- 
rian. 

liamellibraiichs   and   Gasteropods. — The    ordinary 
bivalve-shells  (Lamellihranchs),  and  the  univalves  or  gas- 


FiG.  189.— Silurian  brachio- 
pods; Orthis  Davidsonii. 


I 


PALEOZOIC  ROCKS  AND  ERA. 


281 


teropods,  like  conclis,  whelks,  etc.,  are  also  found;  but, 
in  order  to  avoid  confusing  the  mind  with  too  many  de- 


FiG.  190.— Silurian  brachiopods :  Spirifer  Cumberlandiae— a,  ventral  valve ;  6,  suture. 

tails,  we  shall  pass  over  these  and  confine  ourselves  only 
to  the  most  striking  and  characteristic  forms. 

Cephalopocls ;  Ortlioceratitc. — The  great  class  of 
Cephalopods,  including  now  the  squids,  cuttle-fishes,  and 
nautilus,  were  represented,  in  Silurian  times,  by  a  very 
remarkable  family  called  Orthoceratite  (straight-horn). 
The  appropriateness  of  the  name  is  recognized  by  the 
figures  on  page  382  (Figs.  192,  193). 

Cephalopods  now  are,  some  of  them,  naked  (squid  and 
cuttle-fish) ,  and  some  shelled  (nautilus).     When  they  have 
a  shell,   the   shell   is   cham- 
hered.     The  animal  lives 
in  the  outer  part,  and  all  the 
chambers    are    empty,    full 
of  air  only,  and    connected*^ 
with   the  animal  by  a  mem- 
branous  tube  called  the  si_ 
phon-tube  or  siphuncle  (Fig. 
191).     Now,  at  the  present 
time,  nearly  all  cephalopods 
are  naked.     Only  one  genus 
of  the  shelled  kind  remains, 
viz.,  the  Nautilus.     Tn  Silu- 
rian times,  and  indeed  long  after,  there  were  no  naked 
ones.      Only   the    shelled    kinds    existed.      The    naked 


Fig.  191.— Pearly  luu.ului:'  (Nautilus 
pompilius) :  o,  mantle  ;  6,  its  dor- 
sal fold  ;  c,  hood  ;  o,  eye  ;  t,  ten 
tacles  ;  /,  funnel. 


282 


HISTORICAL   GEOLOGY, 


kinds  are  the  higher.  Again,  now,  and  throughout  all 
later  geological  times,  all  the  shelled  cephalopods  were 
coiled,  like  the  nautilus.  But  Paleozoic,  and  especially 
Silurian  times,  were  characterized  by  the  abundance  of 
long,  tapering,  straight,  chambered  shells.  These  are 
the  Orthoceratites,     They  are  entirely  characteristic  of 


Fig.  192.  Fig.  193.  Fig.  194. 

Pigs.  19S-194.— Silurian  cephalopods  :  192.  Orthoceras  multicameratum.  (After 
Hall.)  193.  Orthoceras  Duseri.  (After  Hall.)  194.  Restoration  of  orthoceras, 
the  shell  being  supposed  to  be  divided  vertically,  and  only  its  upper  part  being 
shown — a,  arms  ;  /,  muscular  tube  ("  funnel  ")  by  which  water  is  expelled  from 
the  mantle-chamber  ;  c,  air-chambers  ;  5,  siphuncle.    (After  Nicholson.) 


Paleozoic,  most  abundant  in  the  Silurian,  and  easily  rec- 
ognized by  any  one.  We  give  figures  of  a  few  (Figs. 
192-194),  and  an  attempted  restoration  of  the  front  part 
of  the  shell  containing  the  animal. 


PALEOZOIC  ROCKS  AND  ERA, 


283 


Orthoceratites  were  extremely  abundant  in  Silurian 
times,  and,  in  some  cases,  reached  an  enormous  size.  In 
the  Silurian  of  the  Western  States,  specimens  have  been 
found  which  were  eight  to  ten  inches  in  diameter,  and 
over  fifteen  feet  long.  They  were  the  most  formidable 
animals  of  these  early  seas.  They  came  in  with  the  Pri- 
mordial, reached  their  maximum  in  the  Mid-Silurian,  but 
continued  through  the  Paleozoic^  and  then  passed  away 
forever. 

Although  the  straight,  chambered  shells  were  by  far 
the  most  abundant,  yet  the  coiled  kinds  were  also  found, 

Crustacea;  Trilobites. — Passing  over  the  worms,  as 
being  of  less  importance,  although  their  borings,  their 
tracks,  their  calcareous  tubes,  and  even  their  teeth,  have 
been  found,  we  come  at  once  to  perhaps  the  most  abun- 
dant and  characteristic  of  all  Paleozoic  forms — Trilobites. 

Description. — The  shell  of  these  curious  creatures 
was  convex  above  and  flat  or,  more  probably,  concave 
below  (Fig,  196).  It  was  divided,  like  most  crustaceans, 
into  many  movable  joints,  but  several  front  joints  were 
always  consolidated  to  make  a  buckler,  or  head-shield, 
and  usually,  but  not  always,  several  hind  joints  were  con- 
solidated to  form  a  pygidium,  or  tail-shield,     Longitudi- 


Fis.  195.— Structure  of  theeye  of  trilobites:  a,  Dalmaniapleuropteryx;  6,  eye  slightly 
magnified;  c,  eye  more  highly  magnified.    (After  Hall.) 


284 


HISTORICAL  GEOLOGY. 


nally,  the  upper  surface  of  the  shell  was  divided  by  two 
depressions  into  three  lobes  (hence  the  name).     On  each 

side  of  the 
head-shield,  in 
position  exact- 
ly as  in  the 
Mng-crah  (Li- 
mulus),  were 
placed  the  eyes; 
and,  strange  to 
say,  we  find  the 
eye,  even  at  this 
early  time,  al- 
ready a  com- 
plex structure 
well  adapted  to 
form  an  image 
(Fig.  195). 
Recently  have 
been  discovered 
jointed  legs, 
ly  I,  each  with 
two  branches, 
one  for  crawl- 
ing and  one  for 
swimming;  and 
also  slender, 
many-jointed 
antennae,  a,  a, 
like  many  crus- 
taceans of  the 
present  day  (Fig.  196).  They  had  the  habit,  which 
many  crustaceans  now  have,  of  folding  themselves  so  as 
to  bring  head  and  tail  together  in  front,  as  shown  in  Fig. 
..jO.  In  the  following  figures  (Figs,  197,  198}  we  give 
some  examjjles  of  Silurian  Trilobites. 


-Restoration  of  upper  side  of  calymene.    (After 
Beecher.) 


PALEOZOIC  ROCKS  AND  ERA. 


mb 


Trilobites  are  found  in  great  numbers,  of  almost  infinite 
variety  of  form  and  markings,  and  of  size  varying  from  a 
fraction  of  an  inch  to  twenty  inches  in  length  (Fig.  197). 


Eto.  197. 


Fig.  198. 


Fig.  199. 
FiG8.  197-199.— Silurian  trilobites-  197.  Paradoxides  Harlani    x  |  (after  Rogers) 
American.    198   Calymene  Blumenbachii.    199.  Same  in  folded  condition. 

They  come  in  with  the  earliest  Primordial,  reach  their 
maximum  in  Mid-Silurian,  but  continue  through  Palae- 
ozoic, and  pass  out  forever.  They  are,  therefore,  entirely 
characteristic  of  the  Paleozoic,  and  especially  abundant 
in  Silurian.  Although  belonging  to  a  distinct  order, 
different  from  any  now  living,  yet  they  were  more  nearly 


286  HISTORICAL  OEOLOOY. 

allied  to  the  horseshoe  crab  (Limulus)  than  anything  else. 
They  are  so  abundant,  so  well  preserved,  and  so  easily  rec- 
ognized, even  by  the  untrained  eye,  that  they  are  a  very 
valuable  means  of  identifying  Paleozoic,  and  especially 
Silurian,  strata. 

Anticipations  of  the  Next  Age, — The  most  highly 
organized  and  most  powerful  animals  of  Silurian  times 
were  undoubtedly  the  OrtJioceratites  and  the  Trilohites. 
The  Orthoceratites  especially  were  the  tyrants  and  scaven- 
gers of  those  early  seas  ;  yet,  in  the  uppermost  Silurian 
are  found  a  few  insects,  scorpions,  and  cockroaches,  and  a 
tew  fishes  similar  to  forms  far  more  abundant  in  Devonian. 
It  is  better,  therefore,  to  regard  these  as  anticipations. 

Section  III. — Devoi^iak  System.    The  Age  of  Fishes. 

Rock-System  ;  Name. — The  name  Devonian  was  given 
to  these  rocks  because  first  studied  with  success  in  Devon- 
shire. In  Scotland  they  were  called  Old  Eed  Sandstone, 
by  Hugh  Miller.  In  England  it  is  often  unconformable 
on  the  Silurian,  but  in  the  Eastern  United  States,  as  al- 
ready stated,  the  Paleozoics  are  conformable  throughout. 
Nevertheless,  even  in  America  there  is  a  great  change  of 
life-forms  at  this  point  of  time  ;  and,  moreover,  the  first 
introduction  here  of  a  new  reigning  class — viz.,  fishes, 
and  a  new  great  department  of  animals — viz.,  Vertebrata, 
or  backboned  animals,  is  a  prodigious  advance,  and  en- 
titles this  to  be  considered  a  distinct  age.  It  is  well  to 
note,  however,  that  some  anticipations  of  this  great  ad- 
vance are  found  in  the  Silurian. 

Area  in  the  United  States. — By  examining  the  map 
on  page  272,  it  will  be  seen  that  in  general  the  Devonian 
rocks  border  on  the  Silurian  area  on  the  south  and  west 
and  extend  far  south  in  the  middle  region.  In  the  Rocky 
Mountain  region  there  are  considerable  areas  of  Devonian, 
but  their  limits  are  too  little  known  to  be  described. 


PALEOZOIC  BOCKS  AND  ERA.  287 

Physical  Geography. — The  land  during  early  Devo- 
nian times  was  the  Archaean  area,  increased  by  the  addi- 
tion of  the  Cambrian  and  Silurian  area,  the  Devonian  area 
being  then  of  course  sea-bottom.  In  the  middle  of  the 
Devonian  Sea  there  was  a  large  island  of  Silurian  rocks 
occupying  what  is  now  mid-Ohio  and  running  down 
through  mid-Tennessee.  At  the  end  of  the  Devonian, 
the  Devonian  area  was  exposed  as  land  and  added  to  that 
previously  existing. 

Subdivisions. — The  American  Devonian  is  subdivided 
into  at  least  four  groups  of  strata  representing  four  periods, 
as  shown  in  the  schedule.  We  shall,  however,  neglect 
these  subdivisions  in  our  general  account  of  the  life- 
system  : 

4.  Chemung  period. 
8.  Hamilton  period. 
2.  Corniferous  period. 
1.  Oriskany. 

Life- System  of  Devonian, — Plants, 

In  Silurian  times,  with  the  exception  of  a  very  few 
small  vascular  cryptogams  allied  to  club-moss,  we  found 
nothing  higher  than  fucoids.  In  addition  to  these,  now, 
for  the  first  time,  land-plants  become  conspicuous.  Here, 
for  the  first  time,  we  have  a  true  forest  vegetation.  The 
character  of  the  trees  of  this  first  forest  is  a  question  of 
the  highest  interest.  The  Devonian  forests  consisted  of 
the  highest  cryptogams,  vascular  cryptogams,  and  the 
lowest  phenogams,  Gymnosperms.  More  explicitly,  there 
were  Ferns,  Lepidodendrids,  and  Sigillarids  (gigantic 
club-mosses),  and  Catamites  (gigantic  Equisetce)  among 
vascular  cryptogams  :  and  Conifers  allied  to  the  yews 
among  gymnospermous  phenogams. 

We  shall  not  describe  these  now,  since  they  are  all 
much  more  abundantly  represented  in  the  Carboniferous. 


288 


HISTORICAL   OEOLOOr. 


We  shall  therefore  dismiss  them  for  the  present  with  one 
or  two  remarks. 

1.  In  Nova  Scotia,  in  direct  connection  with  the  plant- 
beds,  have  also  been  found  many  fossil  forest-grounds. 
These  are  marked  by  dark  seams  with  stumps  and  roots 
in  place  just  as  the  trees  grew.  In  some  cases,  also,  thin 
seams  of  coal  lie  upon  the  forest-grounds.  Thus,  there- 
fore, we  have  here  in  the  Devonian  an  anticipation  not 
only  of  coal  vegetation,  but  also  of  the  conditions  neces- 
sary for  the  formation  and  preservation  of  coal. 

2.  We  have  here  a  somewhat  sudden  appearance  of 
land-plants,  as  if  they  came  without  progenitors.  But 
we  must  remember  that  we  have  a  feeble  beginning  of 
land-plants  in  the  Silurian.  It  seems  probable  that  in 
the  Devonian  we  had  more  favorable  conditions,  and 
therefore  a  rapid  development  of  new  forms. 

Animals. 

If  we  bear  in  mind  what  we  said  about  Silurian  ani- 
mals, it  will  be  necessary  here  only  to  note  the  great 
changes,  i.  e.,  what  old  forms  pass  out,  what  new  forms 
come  in,  and  what  advances  are  made  in  the  progress  of 
life,  dwelling  only  on  the  great  characteristic  of  the  age, 
viz.,  the  flshes. 


Fig.  200. 
Figs.  800,  201.— Devonian  corals 

Wortheni.    (After  Meek.) 


Fio.  201. 
200,    Pavosites  hemispherica.    201.  Zaphrentie 


PALEOZOIC  ROCKS  AND  ERA. 


289 


Radiates. — Among  corals,  the  characteristic  Silurian 
chain-corals  [Halysitids)  are  gone,  but  the  other  two 
forms  remain,  with  different  species  (Figs.  200,  201). 
The  graptolites  are  gone,  as  also  the  Cystidean  crinoids, 
but  the  blastids  or  bud-crinoids  arb  now  far  more  abun- 
dant, though  they  reach  their  maximum  only  in  the 
Carboniferous  (Fig.  242,  page  316). 

Bivalves  and  Univalves. — Brachiopods  still  continue 
in  great  numbers,  of  the  characteristic  Paleozoic,  square- 
shouldered  forms  (Fig.  202),   and  both   Lamellibranchs 


Fig.  202. 


Fig.  203.  Fig.  204.  Fig  205.  . 

Figs.  202-205.— Devonian  brachiopods  :  202.  Spirifer  perextensus.  (After  Meek.) 
Devonian  lamellibranchs  and  gasteropoda  :  203.  Ctenopistha  antiqua.  (After 
Meek.)  204.  Luciua  Ohioensis.  (After  Meek.)  205.  Orthonema  Newberryi. 
(After  Meek.) 


and  Gasteropods  (univalves)  are  now  more  abundant.  It 
is  well  to  note  that  fresh-water  and  land  forms  are  now 
for  the  first  time  introduced. 

Cephalopods. — The  Orthoceratites  still  continue  in 
Devonian  times,  though  in  greatly  diminished  number 
and  size  ;  but  we  note  here  a  great  advance  in  the  intro- 
duction of  a  new  form  characteristic  of  this  and  the 
Carboniferous,  viz.,  the  Goniatites  (angled  stones),  so 
called  because  the  suture  ov  junction  of  the  partition  with 
the  shell  is  angled  instead  of  simple  (see  Fig.  246,  page 

Le  Conte.  Geol.  19 


290 


HISTORICAL   GEOLOGY. 


;317).  It  should  be  remembered  that  this  is  the  first 
introduction  of  a  family  [Ammonite  family)  which  in 
Mesozoic  times  became  extremely  abundant.  The  family 
is  characterized  by  the  dorsal  position  of  the  siphon-tuhe 
and  the  complexity  of  the  suture.  We  shall  hereafter 
trace  the  increasing  complexity  of  the  suture.  It  only 
begins  in  the  Goniatite. 

Crustacea. — Trilobites  still  continue  under  new  forms 
(Fig.  206),  but  in  greatly  diminished  number  and  size. 
They  have  passed  their  prime. 

Insects. — Insects  are  now,  and  at  all  previous  geologi- 
cal times  have  been,  closely  related  to  land  vegetation. 


FiG.  aor. 


Fig.  206. 

Figs.  206,  207.— 206.  Devonian  triloblte  and  inBCCt :   Dalmania  punctata;  Europe. 
207.   Wing  of  platepheraera  antiqua  ;  Devonian,  America.    (After  Dawson.) 


The  first  conspicuous  land  vegetation  is  found  in  the 
Devonian,  and  in  connection  with  this  vegetation  are 
found  also  the  first  known  insects  (Fig.  207).  These  first 
insects  were  most  nearly  allied  to  cockroaches  and  dragon- 
flies — in  fact,  a  connecting  link  between   these  orders. 


PALEOZOIC  ROCKS  AND  ERA.  291 

In  some  a  chirping  organ  has  been  found.  This  sncws 
ihat  an  auditory  apparatus  was  already  developed. 

Although  these  first  known  insects  are  among  the  lower 
Drders  of  the  class,  and  also  are  connecting  links  between 
two  such  lower  orders,  yet  their  somewhat  perfect  devel- 
opment indicates  that  we  must  look  for  the  very  first 
insects  still  lower,  i.  e.,  in  the  Upper  Silurian.  They 
have  been  recently  found  there. 

Fishes. — The  introduction  of  fishes  must  be  regarded 
as  a  great  step  in  the  progress  of  life,  for  it  is  the  begin- 
Qing  not  only  of  a  new  and  higher  class,  but  of  a  new 
great  department  and  the  highest,  viz.,  Vertebrata.  They 
commenced  first  in  the  lowest  Devonian  or  perhaps  even 
in  the  uppermost  Silurian,  few  in  numbers,  small  in  size, 
and  of  strange,  un-fish-like  forms,  but  soon  developed 
in  size  and  numbers  until  these  early  seas  swarmed  with 
them,  and  they  quickly  became  the  rulers  of  the  age. 
The  previous  rulers,  Orthoceratites  and  Trilobites,  there- 
fore diminish  in  number  and  size,  and  thus  seek  safety  in 
subordination.  As  examples  of  the  great  size  of  Devonian 
fishes,  we  mention  a  few.  The  Oncliyodus  (claw-tooth) 
had  jaws  eighteen  inches  long,  armed  with  teeth  two  or 
more  inches  long  ;  the  Dinichthys  (huge  fish)  was  fifteen 
to  eighteen  feet  long,  three  feet  thick,  and  had  jaws  two 
feet  long,  armed  with  curious  blade-like  teeth.  These 
are  from  America.  The  Asterolepis  (star-scale)  of  Europe 
is  believed  to  have  been  twenty  feet  long,  and  of  propor- 
tionate dimensions. 

We  must  not  imagine,  however,  that  these  fishes  were  at 
all  like  most  common  fishes  of  the  present  day.  Neglect- 
ing some  rare  and  unusual  kinds,  fishes  may  be  divided 
into  three  great  orders,  viz.,  1.  Teleosts  (complete  bone); 
2.  Ganoids  (shining)  ;  and  3.  Elasmohranchs  (plate-gills). 
Tlie  'IV'leosts  include  all  the  ordinary  fishes  :  examples  of 
Ganoids  are  found  in  gar-fish  and  sturgeon  ;  of  Elasmo- 
hranchs, in   sharks,  skates,  and   rays.     At   the   present 


292 


HISTORICAL   GEOLOGY. 


time,  nine-tenths  of  all  fishes  are  Teleosts,  but  in  Devo- 
nian times  all  the  fishes  were  Ganoids  and  sharks,  espe- 
cially the  former,  though  differing  in  species  and  genera 
from  Ganoids  and  sharks  of  the  present  day.  But  we 
must  give  some  figures  of  these  strange  Devonian  fishes 
before  discussing  their  affinities  any  further. 

Description  of  Some  I>evoiiiaii  Fishes. — The  Ceph- 
alasjns  (head-shield,  Fig.  208)  was  a  small  fish,  of  very 


Fig.  208. 


Fig.  209. 
Pigs.  208, 209.— Devonian  fishes— Placoderms  :  208.  Cephalaspie  Lyelli. 
olsou.)    209.  Pterichthys  comutus.     (After  Nicholson.) 


(After  Nich- 


curious  shape,  with  mouth  beneath  the  head-shield.  The 
Pterichthys  (winged  fish.  Fig.  209)  was  so  completely  in- 
cased in  bony  plates  that  it  must  have  swum  mainly  by 
means  of  its  wing-like  anterior  fins.  The  mouth  was  also 
beneath.  The  Coccosteus  (berry-bone,  Fig.  210)  was  cov- 
ered with  bony  plates  in  front  parts,  but  the  tail  was 
usable  for  locomotion.     The  Dinichthys  (Fig.  211)  was  a 


PALEOZOIC   ROCKS  AND  ERA. 


293 


huge  fish,  sometimes  eighteen  to  twenty  feet  long,  very 
abundant  in  the  Devonian  of  Ohio.  Like  the  Coccosteus, 
the  anterior  tail  was  covered  with  broad,  bony  plates. 


Fig.  210. 


Fig.  211. 

Figs.  210,  211.— Devonian  fishes— Placoderms  :    210.  Coccosteus  decipiens.    (After 

Owen.)    211.  Dinichthys.    (After  Dean.) 


The  Osteolepis  (bony  scale,  Fig.  212)  was  covered  with  a 
complete  coat-of-mail  of  rhomboidal  bony  scales,  like  the 
gar-fish  and  polypterus  (Fig.  217)  of  the  present  day. 
The  Diplacanthus  (double  spine,  Fig.  213)  is  more  fish-like 
in  form,  but  is  also  covered  with  rhomboidal  bony  scales. 
We  draw  attention  to  the  shape  of  its  tail.  All  these  are 
Ganoids.  The  sharks,  on  account  of  their  cartilaginous 
skeleton  and  imperfect  scales,  are  known  chiefly  by  their 
bony  spines  and  by  their  teeth.  A  restoration  of  a  Devo- 
nian shark  from  Ohio  is  given  in  Fig.  214. 

By  examination  of  the  figures,  it  is  seen  that  Devonian 
Ganoids  are,  some  of  them,  wholly  or  partly  covered  with 
large,  immovable,  bony  plates  (Figs.  208-211)  ;  others 
with  smaller,  rhomboidal,  bony  scales  (Figs.  212,  213). 
The  former  are  called  Placo-ganoids  (plate-ganoids),  or 


294 


HISTORICAL   OEOLOOY. 


Placoderms  (plate-skin);  the  latter,  Lepido -ganoids  (scale- 
ganoids).  Now,  the  Placo-ganoids  are  characteristic  of 
the  Devonian,  and  the  largest  Devonian  fishes,  such  as 


Fig.  212. 


Fig.  214. 
Figs.  212-214.— Devonian  fishes— Lepido-ganoids  :  212.  Osteolepis.    (After  Nichol- 
son.)   213.  Diplacanthus  gracilis.    (After  Nicholson.)    Sharks  :  214.  Cteuacan- 
thus  vetustus,  spine  reduced.    (After  Newberry.) 

the  Dinichthys  and  Aster olepis,  belong  to  this  family. 
The  Lepido-ganoids  continued  after  the  Devonian,  and 
are  still  represented  by  gar-fishes,  etc.  The  sharks  of  the 
Devonian  belong,  all  of  them,  to  a  family  now  almost 
extinct,  called  Oestracionts  {sharp  tool,  referring  to  the 
spine). 

Affinities  of  Devonian  Fishes. — There  are  no  living 
representatives  of  the  Placo-ganoids,  but  there  are  such 
of  the  Lepido-ganoids.  We  herewith  give  figures  of  those 
modern  fishes  which  are  most  like  the  Devonian  fishes. 


PALEOZOIC  ROCKS  AND  ERA. 


295 


The  first  is  an  Australian  fresh-water  fish,  the  recently 
discovered  Ceratodus  (horn-tooth)  (Fig.  215).  The  sec- 
ond, Lepidosiren  (scale-siren),  is  a  very  curious  animal, 
intermediate  between  fish  and  reptile,  found  in  South 
America  and  Africa  (Fig.  216).  The  third  is  the  gar- 
fish, Polypterus  (many  fins),  from  the  Nile  (Fig.  217). 
The  fourth  is  the  only  living  representative  of  cestraciont 
sharks — the  Cestracion  of  Australian  seas  (Fig.  218). 

Bearing  on  Evolution. — It   is   a   curious    fact   that 
these  fishes,  which  are  most  nearly  allied  to  Devonian 


Fig.  216. 

Figs.  215,  216.— Nearest  living  allies  of  Devonian  fishes  :  215.  Ceratodos 

Fosterii,  x  y\j.    (After  Gunther.)    216.  Lepidosiren. 


fishes,  are  by  no  means  low  in  the  scale,  but,  on  the 
contrary,  are,  in  some  respects  at  least,  very  high. 
But  one  thing  is  very  noteworthy,  viz.,  that  they  all 
have  amphibian  characters  united  with  fish  characters 
— they  are  all  connecting  links  letween  fish  and  ampM- 
hian.  For  example,  it  is  seen  that  the  vertebral  col- 
umn in  these,  and  still  more  in  their  Devonian  allies, 
runs  far  into,  often  to  the  end  of,  the  tail-fin.  The 
tail-fin  is  vertebrated.  The  tail  vertebrae  are  finned  on 
the  sides.     This  is  universal  in  Devonian  fishes.     Again, 


296 


HISTORICAL   GEOLOGY, 


it  is  observed  that  in  many  the  paired  fins  are  curiously 
formed — they  are  a  sort  of  \imb^  fringed  with  fin.  Now, 
a  large  number  of  Devonian  fishes  (Fig.  212)  have  this 
style  of  paired  fins.     In  the  third  place,  all  the  living 


Fig,  217. 


Fig.  218. 
Figs.  217, 218.— Nearest  living  allies  of  Devonian  fishes  :  217.  Polypterus.    218.  Ces- 
tracion  Phillippi  (a  living  cestraciont  from  Australia). 


Ganoids  given  above  (Figs.  215,  217)  have  a  more  or  less 
perfect  lung,  and  supplement  their  water-breathing  with 
air-breathing,  in  the  manner  of  some  amphibian  reptiles. 
It  is  almost  certain  that  the  same  was  true  of  Devonian 
Ganoids.  And  yet,  with  all  these  reptilian  characters, 
all  Devonian  fishes  had  cartilaginous  skeletons  like  the 
embryos  of  Teleosts. 

We  wish  now  to  take  advantage  of  these  facts  to  an- 
nounce a  very  general  laAV.  The  first  introduced  exam- 
ples of  any  family,  order,  or  class,  are  not  typical  forms 
of  that  family,  order,  or  class,  but  intermediate  forms  or 
connecting  links  with  other  families,  orders,  etc.  From 
such  intermediate  forms  or  connecting  links  have  been 


FALEOZOIG  ROCKS  AND  ERA.  297 

afterward  developed  the  more  typical  forms.  To  illus- 
trate :  The  first  fishes  were  not  typical  fishes,  but  con- 
necting links  between  fish  and  amphibian,  and  from  this 
intermediate  form,  as  from  a  trunk,  true  fishes  and 
amphibians  were  afterward  separated  and  developed  as 
branches.  Such  intermediate  forms  we  shall  hereafter 
call  generalized  formsr  and  the  more  typical  forms  into 
which  they  seem  to  be  afterward  developed,  specialized 
forms.  We  shall  find  many  illustrations  of  this  law  as 
we  proceed. 

Apparent  Suddenness  of  the  Appearance  of 
Fishes. — At  a  certain  time  fishes  seem  suddenly  to  appear, 
as  if  they  came  without  progenitors.  But  we  must  re- 
member that  the  very  lowest  forms  of  fishes  have  neither 
bony  skeleton  nor  scales,  and  their  remains  are  not  likely 
to  be  preserved.  We  may  yet  find  evidences  of  such  far 
down  in  the  Silurian.  Nevertheless,  there  can  be  little 
doubt  that  conditions  were  favorable  for  the  development 
of  fishes  about  the  beginning  of  the  Devonian,  and  there- 
fore the  steps  of  development  were  exceptionally  rapid  at 
that  time. 

Section  IV. — Carbokiferous  System.     Age  of  Acbo- 
GENS  AND  Amphibians. 

Subdivisions. — The  Carboniferous  age  is  subdivided 
into  three  periods  :  1.  Sub-carhoniferous  ;  %.  Carbonifer- 
ous proper,  or  coal-measures  ;  3.  Permian.  The  first  may 
be  regarded  as  the  preparation,  the  second  the  culmina- 
tion, and  the  third  the  transition  to  the  Mesozoic.  The 
whole  carboniferous  strata  in  Nova  Scotia  is  16,000  feet 
thick,  in  Wales  14,000  feet,  in  Pennsylvania  9,000  feet. 

The  sub-carboniferous  strata  are  mostly  limestones ; 
those  of  tlie  coal-moasures  mostly,  tliough  not  wliolly, 
sands  and  clays.  The  sub-carboniferous  are  marine  de- 
posits,   the    coal-measures   mainly   fresh-water   depositSo 


298  HISTORICAL  GEOLOGY, 

The  fossils  of  the  former  are,  therefore,  marine  inimals  ; 
those  of  the  latter  mainly  land-plants,  and  fresh-water  and 
land  animals.  In  both  Europe  and  America  the  sub- 
carboniferous  underlies  the  coal-measures  and  outcrops 
around,  and  thus  forms  a  penumbral  margin  about  the 
black  areas  representing  coal-fields  on  geological  maps 
(see  Fig.  169). 

After  this  brief  comparison  and  contrast,  we  shall  now 
concentrate  our  attention  on  the  coal-measures,  because 
all  the  characteristics  of  the  Carboniferous  age  culminate 
there.  In  speaking  of  the  fauna,  however,  we  shall  take 
the  two  together.  The  Permian  will  be  treated  as  a 
transition  to  the  Mesozoic. 


Carboniferous  Proper — Rock-system,  or  Coal- Measures, 

Name. — The  Carboniferous  period  is  but  one  of  three 
periods  of  the  Carboniferous  age.  The  Carboniferous  age 
is  but  one  of  the  three  ages  of  the  Paleozoic  era.  The 
Paleozoic  era  is  but  one  of  the  four  great  eras,  exclusive 
of  the  present.  The  Carboniferous  period,  therefore,  is 
but  a  small  fraction,  certainly  not  more  than  one  twentieth 
to  one  thirtieth  of  the  recorded  history  of  the  earth.  Yet, 
during  this  period  were  accumulated,  and  in  its  strata 
were  preserved,  and  are  now  found,  nine  tenths  of  all 
the  coal  used  by  man.  The  name  carhoniferous,  for  the 
period,  and  coal-measures,  for  the  strata,  is  surely,  there- 
fore, appropriate. 

Thickness  of  the  Strata. — Although  so  small  a  por- 
tion of  the  whole  strata  of  the  earth,  these  coal-measures 
are  often,  locally,  of  great  thickness.  In  Nova  Scotia  the 
coal-measures,  exclusive  of  the  sub-carboniferous,  are 
14,000  feet  thick,  in  Wales  12,000,  in  Arkansas  25,000 
(Branner),  and  in  West  Virginia  5,000. 

Mode  of  Occurrence  of  Coal. — Such  being  the  thick- 
ness, it  is  evident  that  but  a  small  portion  is  coal.     In 


PALEOZOIC  ROCKS  AND  ERA. 


299 


^mm^m 


^Mi 


^m^m 


rJr'rX'J- 


fact,  the  coal-measures  consist  of  alternations  of  sand- 
stones, shales,  and  limestones,  like  other  formations  ;  but 
interbedded  with  these  are  also  seams  of  coal  and  beds 
of  iron-ore.  These  five  kinds  of  strata  alternate  with 
each  other,  and  are  each  repeated  many- 
times,  but  without  any  regular  order,  as 
shown  in  Fig.  219,  which  is  an  ideal 
column  from  a  coal-fieldo  Thus,  the 
strata  of  a  coal-field  may  be  likened  to  a 
ream  of  sheets  of  five  colors,  but  arranged 
without  order.  Only  this  may  be  said, 
that  beneath  every  coal-seam  there  is  al- 
ways a  thin  layer  of  clay,  called  the  under- 
clay^  and  above  is  usually,  but  not  always, 
a  shale,  culled  the  blach  shale  or  roof- 
shale.  It  is  a  rich  coal-measure  in  which 
we  find  one  foot  of  coal  for  fifty  feet  of 
rocko 

Subsequent  Changes. — The  strata  of 
coal-measures,  like  all  other  strata,  were 
horizontal  when  first  laid  down  ;  but,  like 
other  strata  also,  they  have  been  elevated, 
and  tilted  and  folded  and  crumpled  and 
broken  and  faulted,  especially  in  moun- 
tain-regionso  And  in  all  cases,  whether 
horizontal  (Fig.  221)  or  folded  (Fig.  220), 
they  have  been  largely  carried  away  by 
erosion,  and  the  strata  left  outcropping 
on  the  surface  (Figs.  220,  221),  and  often 
in  isolated  patches.  Since  coal-seams,  like  other  strata, 
are  broken  and  faulted,  it  is  very  important  to  remember 
the  law  of  slip  mentioned  on  page  232c 

Thickness  and  Number  of  Seams. — The  thickness 
of  seams  varies  from  a  few  inches  to  many  yards.  The 
mammoth  seam  of  Pennsylvania  is  over  one  hundred  feet 
thick.     The  best  thickness  for  easy  working  is  ab  out  six 


1  t  J  /   M..  i  ■ 


Fig.  219.— Ideal  sec- 
tion showing  alter- 
nation of  different 
kinds  of  strata  :  Ss, 
sandstone ;  Sh., 
shale  ;  /,  limestone  ; 
i,  iron ;  and  c,  coal. 


300 


HISTORICAL   GEOLOGY. 


to  ten  feet.     The  number  of  seams  in  a  single  field  may 
be  a  hundred  or  more,  and  their  aggregate  thickness  may 


Fig.  230.— Panther  Creek  and  Summit  Hill  traverse.    (After  Daddow.) 


Fiu.  221.— Illinois  coal-field.    (After  Daddow.) 


be,  in  rfome  cases,  one  hundred  to  one  hundred  and  fifty 
feet  of  solid  coal. 

Coal-Fields  of  the  United  States. — In  the  map  on 

page  272  the  coal-fields  of  the  United  States  belonging 
to  this  period  are  represented  in  black.  It  is  seen  that 
there  are  four  of  these  :  1.  The  Appalachian  coal-field, 
probably  the  richest  in  the  world.  In  a  general  way  it 
may  be  said  to  cover  the  western  slope  of  the  Appalachian 
chain  from  Pennsylvania  southward.  It  covers  an  area 
of  60,000  square  miles.  2.  Tlie  central  coal-field.  This 
covers  nearly  the  whole  of  Illinois,  the  western  portion  of 
Indiana,  and  northwestern  Kentucky,  and  its  area  is  47,000 
square  miles.  3.  TJie  great  Western  coal-field.  This  cov- 
ers southern  Iowa,  northwestern  Missouri,  eastern  Kan- 
sas, the  Indian  Territory,  western  Arkansas,  and  north- 
ern Texas.  Its  area  is  no  less  than  78,000  square  miles. 
4.    The   Michigan    coal-field.     This   occupies  an  area   of 


PALEOZOIC  ROCKS  AND  ERA. 


301 


6,700  square  miles  in  the  center  of  the  Michigan  Peninsula. 
Besides  these,  there  is  a  small  area  of  coal  of  little  value 
in  Rhode  Island,  and  a  fine 
coal-field  of  18,000  square 


Appalachian 

Central 

Great  Western. 

Michigan 

Rhode  Island. . 


00,000 

47,000 

78,000 

G,700 

500 


miles  accessible  to   us  in 

Nova     Scotia.       Of     the 

192,200    square    nules    of 

coal  within  the  limits  of 

the  United  States,  120,000 

square  miles  are  estimated 

as  workable.     It  may  be 

said  with  confidence  that 

there  is  no  country  in  the 

world  so  liberally  supplied  with  this  great  agent  of  modern 

civilization  as  our  own. 


Nova  Scotia. 
Total 


192,200 
18,000 

210,200 


Origin  of  Coal  and  of  its  Varieties, 

There  can  be  no  doubt  that  coal  is  of  vegetable  origin. 
All  portions  of  a  coal-seam,  even  the  most  structureless 
to  the  naked  eye,  when  properly  prepared,  reveal  their 
vegetable  structure  to  the  microscope  (Figs.  222,  223). 


Fig.  222.— Section  of  anthracite  :  a,  natural  size  ;  b  and  c, 
magnified.  (After  Bailey.) 


Fig.  223.— Vegetable 
stmcture  in  coal. 
(After  Dawson.) 


i 


Varieties  of  Coal. — Assuming  the  vegetable  origin  of 
coal,  how  do  we  account  for  the  varieties  ?  These  varie- 
ties are  of  three  kinds  :  1.   Varieties  depending  on  degrees 


302  HISTORICAL   OEOLOOY. 

of  purity  ;  2.  On  degrees  of  bituminization  ;  3.  On  the 
relative  proportion  of  fixed  and  volatile  matter. 

1.  Varieties  depending'   on  Degrees  of  Purity. — 

Coal  consists  of  combustible  and  incombustible  matter, 
or  ash.  The  combustible  matter  is  organic,  the  ash  min- 
eral. Now,  the  relative  proportion  of  these  varies  in 
every  degree.  The  purest  coal  may  contain  only  1  to  2 
per  cent,  of  ash ;  but  coal  may  contain  5  to  10  per  cent., 
20  to  30  per  cent.,  50  to  60  per  cent.,  and  so  on  to  90,  95, 
99  per  cent.  ash.  If  a  coal  contains  not  more  than  5  per 
cent,  ash,  it  is  probably  pure,  i.  e.,  its  ash  is  wholly  due 
to  ash  of  original  vegetable  matter ;  but  if  it  contains 
more  than  10  per  cent.,  it  is  certainly  impure,  the  excess 
being  due  to  mud  deposited  with  the  vegetable  matter. 

2.  Varieties  depending  on  the  Degrees  of  Bitu- 
minization.— Coal  may  be  pure,  and  yet  imperfectly 
bituminized.  Such  are  lignites,  hrown  coal,  and  the  like. 
This  depends  mainly  on  age,  the  oldest  coals  being  most 
completely  changed. 

3.  Varieties  depending-  on  the  Relative  Propor- 
tion of  Fixed  and  Volatile  Matter. — In  pure  and  per- 
fect coal  there  are  still  varieties  depending  on  the  relative 
amount  of  fixed  carbon  and  volatile  hydrocarbon,  and  it 
is  mainly  this  which  produces  the  varieties  of  good  coal, 
and  determines  its  various  uses.  If  the  coal  contains  only 
5  to  10  per  cent,  volatile  matter,  it  i&  called  anthracite, 
which  is  a  hard,  lustrous  variety,  breaking  with  a  con- 
choidal  fracture,  and  burning  with  very  little  blaze,  but 
great  heat.  If  it  contains  15  to  20  per  cent,  of  volatile 
matter,  it  is  called  semi-bituminous,  or  steam-coal,  because 
of  its  excellence  in  rapid  formation  of  steam.  It  burns 
with  a  long  blaze,  but  does  not  caTce.  If  it  contains  30 
to  40  per  cent. ,  it  is  ordinary  bituminous  caking  coal ;  if 
50  per  cent.,  or  more,  highly  bituminous,  fat,  or  fusing 
coal.  In  this  series  we  might  well  put  graphite,  or  plum- 
bago, above  anthracite  ;  for  graphite  consists  of  carbon 


PALEOZOIC  ROCKS  AND  ERA,  303 

without  any  volatile  matter,  and,  although  it  is  not  called 
coal,  because  incombustible,  yet  it  is  but  the  last  term  in 
the  above  series  of  varieties. 

Cause  of  these  Varieties. — Vegetable  matter  decay- 
ing out  of  contact  with  air,  i.  e.,  beneath  water  or  buried 
in  mud,  loses  a  large  portion  of  its  material  in  the  form  of 
gases  (CO,,  CH,,  and  H,0).  These  (CO,  and  CHJ  are 
the  gases  which  escape  in  bubbles  when  we  stir  the  bottom 
of  a  stagnant  pool  in  which  plants  are  growing.  They 
are  also  the  gases  which  are  constantly  escaping  in  every 
coal-mine,  and  form  the  deadly  chohe-damp  and  the  still 
more  dreaded  ^re-c?am/?  of  the  mines.  Now,  the  relative 
proportion  in  which  these  are  given  off  determines  most 
of  the  above  varieties. 

Anthracite  and  graphite  may  be  regarded  as  metamor- 
phic  coals.  The  reasons  for  so  thinking  are  mainly  the 
following  :  1.  Coal  is  often  made  locally  anthraeitic  by 
a  lava-flow  or  dike.  2.  In  the  same  coal-field,  wherever 
the  strata  are  crumpled  and  metamorphic,  as  in  eastern 
Pennsylvania,  the  coals  are  anthraeitic  ;  and  where  the 
strata  are  flat-lying  and  unchanged,  as  in  Ohio,  the  coal 
is  bituminous. 

Plants  of  tJie  Coal, 

In  no  other  strata  have  the  remains  of  plants  been 
found  in  so  great  abundance  and  variety  as  in  the  coal- 
measures.  We  could  expect  nothing  else  when  we  re- 
member that  a  coal-seam  is  a  mass  of  vegetable  matter, 
and  that,  on  account  of  their  economic  value,  these  seams 
are  continually  explored.  The  remains  of  plants  are 
found  in  the  form  of  leaves,  flattened  stems  and  branches, 
and  sometimes  fruits,  in  connection  with  the  black  roof- 
shale  ;  and  as  stumps  and  roots,  in  connection  with  the 
under-day  or  floor  of  the  seams. 

Principal  Kinds. — The  plants  belong  mainly  to  four 
or  five  great  orders,  viz.,  Conifers  and  probably  Cycads, 


304 


HISTORICAL   GEOLOOY. 


among  gymnosperms,  and  Ferns,  Clnh-mosses,  and  Equi- 
setce,  among  vascular  cryptogams.  These  orders  were 
anticipated  in  the  Devonian,  but  culminate  here. 

1.   Conifers  and  Cycads. — These  are  found  as  leafy 


^^^^-^'^^^^^ 


Pig.  224.— Araucarites  tn-acilis,  reduced.  Fig,  225.— Cordailes.    (Restored 

(After  Dawson.)  by  Dawson.) 


Fig.  226. 


Fig.  2:>7. 


Fig.  228. 


Tios.  226-228.— Fniita  of  coal-plants,  probablj^  conifers  :  Cardiocarpon.    (Afta 
Newberry  aud  Dawson.) 


PALEOZOIC  BOCKS  AND  ERA. 


305 


branches  (Fig.  22-1:),  as  scattered  leaves,  like  those  in  the 
restored  tree  (Fig.  225),  as  nut-like  fruits  (Figs.  226-228), 
near  the  top  of  the  coal-seams,  and  sometimes  as  drift- 
logs  in  the  sandstones,  interstratified  with  the  coal.  The 
trunks  are  known  to  be 
conifers  by  the  microscopic 
structure  of  the  wood,  the 
cells  of  which  are  marked 
with  circular  disks  on  lon- 
gitudinal section  (Fig.  229), 
and  on  cross-section  the 
wood  is  destitute  of  pores. 

Now,  what  kind  of  coni- 
fers have  such  leaves  and 
fruit  as  these  ?  None  but 
the  yew  family.  All  of  these 
have  plum-like  fruit  with 
nut-like  seeds,  and  many  of 

them  have  broad  leaves  (Fig.  230).     The  cordaites  (Fig. 
225)  has  been  found  with  trunk   sixty  to   seventy  feet 


Fig.  229.— Longitudinal  section  of  wood 
of  a  living  conifer,  magnified. 


Fig.  230.— Living  broad-leaved  yews. 
Lb  Conte,  Gkol.  20 


30C 


HISTORICAL   QKOLOOY. 


long,  crowned  with  broad  leaves,  and  with  a  spike  of  fruit. 
It  is  probably  a  Cycad,  or  else  a  broad-leaved  conifer  like 
the  ginkgo. 

2.  Ferns. — These  are  the  most  abundant,  but  not  the 
largest,  plants  of  the  coal.  About  one  half  of  all  the 
known  species  of  coal-plants  are  ferns.  They  are  often 
beautifully  preserved,  large,  complex  fronds  spread  out 
and  pressed,  as  if  between  the  leaves  of  a  botanist's  her- 
barium, with  even  the  microscopic  veining  of  leaflets 
visible.  They  are  known  to  be  ferns — 1.  By  their  large, 
complex  fonds  (Fig.  231).     2.  By  the  peculiar  veining  of 


Fig.  231.  Fig.  232.  Fig.  288. 

Figs.  231-2.33.— Coal-ferns:  231.  Megaphyton,  a  coal-fern  restored.  (After  Dawson.) 
232.  Callipteris  Sullivanti.  (After  Lesquereux.)  233.  PecopterisStrongii.  (After 
Lesquereux.) 

the  leaves,  characteristic  of  ferns  (Fig.  232).  3.  By  the 
rows  of  spore-cases  on  the  under  surface  of  the  leaves 
(Fig.  234).  4.  In  the  case  of  tree-ferns,  by  ragged,  ovoid 
marks,  leaf-scars  left  by  the  fallen  fronds.  We  give  a  few 
figures  of  ferns  of  the  American  and  French  coal-measures. 
The  remaining  orders,  viz.,  Lycopods  (or  club-mosses) 
and  EquisetcB  (horse-tails  or  scouring-rushes),  are  still 
more  important,  for  two  reasons  :  1.  They  formed  the 
principal  mass  of  the  coal.  2.  They  were  very  remarkable 
examples  of  generalized  types  or  connecting  links,  and 


PALEOZOIC  ROCKS  AND  ERA, 


307 


Fig.  a34.— Dactylothe> 
ca  dentata.  (After 
Zeiler,)  a,  spore  case 
enlarged. 


possess  a  liigh  interest  on  that  account. 
We  shall  treat  of  them  under  three 
heads,  viz.,  LepidodeJidrids,  Sigilla- 
ridSj  and  Calamites. 

1.  Lepidodeiidrids. — Every  part  of 
these  has  been  found — roots,  stems, 
branches  clothed  with  leaves  and  tipped 
with  fruit.  They  may  be  restored, 
therefore,  with  some  degree  of  confi- 
dence. Imagine,  then,  a  trunk  two, 
three,  or  even  four  feet  in  diameter  at 
its  base  where  it  joins  the  wide-spread- 
ing roots;  marked  with  regular  rhomboidal  figures,  which 
are  the  leaf-scars  (Fig.  236)  ;  branching  widely,  but  not 

profusely ;  the  great  branches, 
clothed  with  scale-like  or  needle- 
like leaves,  stretching  aloft,  like 
uplifted  hairy  arms,  to  the 
height  of  fifty  or  sixty  feet,  and 
terminating  in  scaly  cones  like 
club-mosses.  The  most  common 
findings  are  flattened  stems  with 
beautiful  rhomboidal  markings 
(Fig.  236),  looking  much  like 
rhomboidal  scales  of  a  ganoid 
fish ;  hence  tKe  name  Lepido- 
dendron,  or  scale-tree. 

There  can  be  no  doubt  that 
the  Lepidodendron  was  a  lyco- 
pod,  or  club-moss  ;  but  its  inter- 
nal structure,  as  well  as  its  great 
size  (club-mosses  are  now  but  a 
few  inches,  or,  at  most,  a  few 
feet  high),  ally  it  strongly  with 
conifers.  We  may  regard  it,  therefore,  as  a  lycopod,  with 
characters  connecting  it  with  conifers. 


Fig.  235. — Restoration  of  a  Lepi- 
dodendron,    (By  Dawson.) 


308 


EISTORICAL  GEOLOGY. 


2,  Sig-illarids. — The  family  name  is  taken  from  the 
type  genus,  Sigillaria.  It  includes  Sigillaria  and  Sigil- 
larialike  plants.     The  name  Sigillaria  isi  taken  from  the 


iMfl 

mmm 


Pig.  236.  —  Lepidodendron  Fig.  237.  —  Sigillarids  :  Sigil-  Fig.  338.— Kestora- 
modulatum.  (After  Les-  laria  reticulata.  (After  Les-  tion  of  Sigillaria. 
quereux.)  quereux.)  CBy  Dawson.) 

seallike  markings  (sigilla,  a  seal)  left  on  the  trunk  by 
the  falling  leaves  (Fig.  237).  They  were  the  largest  of 
all  the  coal-trees.  Eoot,  stem,  branches,  and  leaves  have 
been  found.  From  these  it  is  possible  to  reconstruct  the 
g'^.neral  appearance  of  :he  tree.  Imagine,  then,  a  tree 
four  or  five  feet  hi  diameter  at  the  base,  with  widely 
spreading  roots  ;  tne  trunk  regularly  fluted  like  a  Corin- 
thian column,  and  ornamented  with  vertical  rows  of  seal- 
like impressions  (leaf-scars),  and  towering  to  the  height 
of  one  hundred  to  one  hundred  and  fifty  feet ;  the  top 
branchless,  or  else  with  only  a  few  large  branches  clothed 
with  grasslike   or  yuccalike    leaves.     The  fruit  is  not 


PALEOZOIC  ROCKS  ANU  ERA. 


309 


known  with  certainty.     The  general  appearance  is  given 
in  Fig.  238. 

3.  Calamites. — These  are  so  named  from  their  jointed, 
reed-like  appearance  {cala- 
mus,  a  reed).  They  are  usu- 
ally found  in  the  form  of  flat- 
tened, jointed,  and  striated 
stems.  They  may  be  de- 
scribed as  follows :  Imagine 
a  straight^  hollow,  jointed, 
tapering  stem^  one  to  two  feet 
in  diameter,  and  twenty, 
thirty,  or  forty  feet  high,  ter- 
minating in  a  compact,  cone- 
like fruit  (Fig.  240),  the  joints 
striated,  but  the  grooves  in- 
terrupted at  the  joints  by 
whorls  of  scale-like  leaves,  or 
else  whorls  of  jointed,  thread- 
like branches  (Fig.  239)  about 
the  joints.  From  the  basal 
joints  come  out  thread-like 
roots.  Fig.  239  is  a  restora- 
tion of  its  appearance. 

Now,  all  that  we  have  said  applies,  word  for  word,  to 
equisetae,  or  horse-tails,  except  the  great  size.  But  equi- 
sett^  of  the  present  day  are  small,  rushlike  or  reedlike 
plants.  Moreover,  the  internal  structure  of  Calamites 
shows  a  close  relation  with  gymnosperms,  probably  coni- 
fers. 

Conclusion. — The  general  conclusion,  then,  is  that  all 
the  plants  of  the  Coal,  but  especially  the  Lepidodendrids, 
the  Sigillarids,  and  the  Calamites,  were  remarkable  gen- 
eralized types,  connecting  classes  and  orders  now  widely 
separated  from  each  other — viz.,  the  higher  or  vasculai 
cryptogams  with  the   lowest  or  gymnospermous  pheno- 


Fig.  239.— Restoration  Fig.  240.— Fruit 
of  a  Calainite.  (Af-  of  Calamite. 
ter  Dawson.)  (After  Heer.) 


310  HISTORICAL  GEOLOGY, 

gams.  The  two  branches  of  the  tree  of  life,  cryptogam 
and  phenogam,  so  widely  separated  now,  when  traced 
downward,  approach  and  almost  meet  here  in  the  Coal 
period. 

Mode  of  Accumulation  of  Goal. 

There  has  been  much  dispute  on  this  subject,  and  it  is 
still  obscure.  There  are  some  things,  however,  which 
are  reasonably  certain.  We  shall  give  what  is  most 
certain,  in  the  form  of  three  propositions  : 

1.  Coal  lias  beeu  accumulated  in  the  presence  of 
water. — This  is  indicated  [a]  by  the  nature  of  the 
plants,  which  are  mostly  swamp-plants  ;  (b)  by  the  inter- 
stratified  sands  and  clays,  which  were,  of  course,  deposited 
in  water  ;  but,  more  than  all  {c)  by  the  preservation  of 
the  vegetable  matter,  which  would  have  entirely  disinte- 
grated and  passed  off,  as  CO,  and  H,0,  unless  completely 
water-soaked. 

2.  Coal  has  been  formed  hy  accumulation  of 
vegetable  matter  "in  place" — ^i.  e.,  where  the  plants 
grew — by  annual  decay  of  generation  after  generation,  as 
we  see  now  in  peat-bogs  and  peat-swamps ;  axid  not  by 
accumulation  by  driftage,  as  we  see  in  rafts.  The  evi- 
dence of  this  is  complete.  We  shall  only  mention  one 
fact,  which  is  demonstrative  :  The  under-day  of  every 
coal  \%full  of  stumps  and  roots  in  position  as  they  grew. 
Every  under-clay  is  an  old  fossil  forest-ground,  or  rather 
swamp-ground. 

Imagine,  then,  an  old  coal-swamp,  with  its  clay  bot- 
tom full  of  dead  stumps  and  roots,  with  its  accumulation 
many  feet  deep  of  pure  peat,  with  its  surface  covered 
with  late-fallen  leaves,  broken  branches,  and  prostrate 
trunks,  and  the  still  growing  vegetation  shading  all. 
Now,  imagine  this  overwhelmed  and  buried  by  sediments, 
subjected  to  powerful  pressure  and  slow  change,  and  we 
have  all  the  phenomena  of  a  coal-seam,  with  its  under- 


PALEOZOIC  ROCKS  AND  ERA.  311 

clay  full  of  roots  and  its  roof-shale  full  of  impressions  of 
leaves  and  flattened  branches,  etc. 

3.  Coal  lias  been  accumulated  at  the  mouths  of 
rivers,  and  therefore  subject  to  overflows  and  deposits  of 
mud  by  the  river,  and  to  occasional  incursions  by  the  sea. 
This  is  proved  by  the  alternation  of  river-sand  and  clay 
with  marine  deposits  of  limestone. 

It  may  be  difficult  to  put  these  three  propositions  to- 
gether and  form  a  clear  picture  of  the  precise  manner  of 
accumulation,  and  therefore  there  is  still  a  large  field  for 
the  play  of  fancy. 

Estimate  of  Length  of  the  Coal  Period, 

If  the  sands  and  clays  of  a  coal-field  have  been  accu- 
mulated by  river-deposit,  then  we  have  a  means  of  making 
a  rough  approximate  estimate  of  the  time  embraced  by 
the  Coal  period.  It  is  true,  agencies  may  have  acted 
then  at  a  different  rate  from  now,  but  our  estimate  will 
be  liberal. 

For  this  purpose  we  take  the  Nova  Scotia  coal-field, 
because  the  evidence  of  river-deposit  is  very  strong  in 
every  part.  It  has  been  estimated  that  there  were  not 
less  than  54,000  cubic  miles  of  river  sediment  in  the 
original  field.  Now,  the  Mississippi  River  at  present  ac- 
cumulates one  twentieth  cubic  mile  per  annum,  and 
would  therefore  take  twenty  years  to  accumulate  one 
cubic  mile,  and  1,080,000  years  to  accumulate  54,000 
cubic  miles.  But,  as  already  said  (page  298),  the  Coal 
period  is  but  a  small  fraction,  certainly  not  more  than 
one  twentieth  to  one  thirtieth,  of  the  recorded  history  of 
the  earth.  Therefore,  this  recorded  history  can  not  be 
less  than  twenty  to  thirty  millions  of  years.  It  is  proba- 
bly much  more.  AYe  only  give  this  estimate  in  order 
to  accustom  the  mind  to  the  great  periods  of  time  with 
which  geology  deals. 


ai2  HISTORICAL   GEOLOGY, 


Physical  Geography  and  Climate  of  the  Coal  Period. 

Physical  Geography. — The  Paleozoic  era  was  a  time 
of  gradual  growth  of  the  continent  from  the  Archaean 
nucleus  by  successive  additions,  first  of  the  Silurian,  then 
of  the  Devonian,  and  now  of  the  Carboniferous  area. 
During  Carboniferous  times  the  form  of  the  American 
Continent  probably  did  not  differ  greatly  from  that  repre- 
sented on  page  349  (Fig.  303)  as  the  Cretaceous  conti- 
nent, except  that  the  areas  of  coal-measures  were  not 
then  permanent  land,  but  were  in  an  uncertain  state, 
sometimes  swamp-land,  sometimes  covered  with  river- 
sediment,  sometimes  covered  by  the  sea.  Although  the 
continent  had  greatly  grown,  still  we  must  imagine  it 
as  small  and  low  compared  with  its  present  state.  The 
same  is  probably  true  of  other  continents. 

Climate. — The  climate  was  probably  warm,  very  moist, 
very  uniform,  and  the  air  loaded  with  CO^.  The  greater 
warmth  and  uniformity  are  shown  by  the  fact  that  the 
plants  are  those  of  a  tropical  climate.  Tree-ferns,  arbo- 
rescent lycopods,  etc.,  grew  then  with  ultra-tropical  lux- 
uriance, not  only  in  now  temperate  regions,  but  in  Mel- 
ville Island  and  Grinnell  Land,  78°-80°  north  latitude. 
The  prevalence  of  the  great  coal-swamps  and  the  charac- 
ter of  the  plants  are  sufficient  evidence  of  greater  humid- 
ity. Finally,  Avhen  we  femember  that  the  whole  of  the 
coal  in  the  world  represents  so  much  carbon  taken  from 
the  atmosphere,  as  CO^  with  return  of  the  oxygen,  we 
shall  be  convinced  that  the  quantity  of  CO,  in  the  air 
was  greater  and  of  oxygen  less  than  now. 

It  is  probable,  therefore,  that  in  early  geological  times 
there  were  more  moisture  and  CO,  and  less  oxygen  than 
now.  This  would  make  a  paradise  for  plants,  especially 
the  lower  orders,  but  would  be  unsuitable  for  air-breath- 
ing animals.     There  has  been  throughout  all  geological 


PALEOZOIC  ROCKS  AND  ERA.  313 

times  a  gradual  purification  of  the  air  of  its  superabun- 
dant moisture  by  increase  of  the  size  and  height  of  con- 
tinents, and  of  its  superabundant  00^  by  its  withdrawal 
in  many  ways,  but  during  the  Coal  period  especially  by 
the  growth  of  plants  and  the  preservation  of  the  carbon 
as  coal.  In  this  process  not  only  was  the  CO^  removed, 
but  oxygen  restored,  and  thus  was  the  air  prepared  for 
the  use  of  air-breathing,  hot-blooded  animals,  such  as 
birds  and  mammals,  which  were  accordingly  introduced 
soon  afterward. 

Petroleum  and  Bitumen, 

We  take  up  these  here,  not  because  they  are  peculiar 
to  the  coal-measures,  for  such  is  not  the  fact,  but  because 
they  seem  to  have  been  formed  from  organic  matter  by 
a  process  similar  to  that  of  coal,  and  also  because  some 
tliink  they  are  actually  formed  from  coal  by  distillation. 
This,  however,  is  not  probable. 

If  bituminous  coal,  or  any  organic  matter,  be  heated 
red-hot,  out  of  contact  with  air,  the  volatile  matters  are 
driven  off,  broken  up,  and  recombined,  and  may  be  col- 
lected in  a  great  variety  of  forms  of  hydrocarbons — some 
solid,  as  coal-pitch  ;  some  tarry,  as  coal-tar  ;  some  liquid, 
as  coal-oil;  some  volatile,  as  coal-naphtha;  and  some 
gaseous,  as  coal-gas.  Now,  a  somewhat  similar  series  of 
hydrocarbons  is  found  in  the  earth  and  issuing  on  its  sur- 
face :  some  solid,  as  asphalt,  Albertite,  Graliamitc,  etc.  ; 
some  tarry,  as  bitumen  ;  some  liquid,  as  petroleum  ;  some 
volatile,  as  rock-naphtha  ;  some  gaseous,  as  the  gas  of 
burning-springs.  It  is  almost  certain  also  that  these  are 
of  organic  origin. 

Mode  of  Occurrence. — Petroleum  occurs  in  the  strata 
much  as  water  docs,  and  the  two  are  often  associated. 
Like  water,  and  with  water,  it  is  found  in  porous  and 
fissured  strata,  such  as  sandstones  and  limestones,  when 
these  are  covered  with  a  stratum  of  impermeable  shale. 


314  HISTORICAL  GEOLOGY. 

Like  water,  and  with  water,  it  often  oozes  on  the  surface 
as  hillside  springs.  With  water,  it  collects  in  fissures 
and  cavities  of  all  kinds,  from  which,  through  artesian 
wells,  it  issues,  in  some  cases,  in  great  quantities  as 
fountains. 

But,  unlike  water,  there  is  no  great,  continuous,  peren- 
nial supply  ;  and  also,  unlike  water,  the  force  by  which 
it  spouts  is  not  by  hydrostatic  pressure  alone,  but  hydro- 
static pressure  transmitted  through  the  elastic  force  of 
compressed  gas  always  associated  with  the  oil.  As  gas, 
oil,  and  water  are  nearly  always  found  together,  these 
arrange  themselves  in  the  order  of  their  relative  specific 
gravities  ;  and  therefore  in  a  flowing  well  the  water  usu° 
ally  appears  only  after  the  gas  and  oil  are  exhausted. 
The  flow  of  oil  wells  is  not  perennial  like  water,  because 
the  oil  is  not  continually  re-supplied.  The  accumulation 
of  ages  is  now  being  exhausted  with  a  rapidity  propor- 
tioned to  the  abundance  of  the  flow.  What  is  true  of 
oil  is  much  more  true  of  gas.  A  gas  well  is  necessarily 
short-lived. 

Age  of  Petroleum-bearing  Strata. — Petroleum  has 
been  found  in  strata  of  nearly  all  ages,  but  under  the 
two  conditions  of  local  abundance  of  the  organic  matter 
from  which  this  substance  is  formed,  and  the  absence  of 
metamorphism,  which  a^lways  changes  it  into  asphalt. 
At  one  time  it  was  supposed  to  be  characteristic  of  newer 
rocks,  having  been  found  in  foreign  localities,  mostly  in 
Tertiary  strata.  But  in  the  Eastern  United  States  it  is 
confined  to  the  Paleozoic  rocks,  while  in  California  it 
is  again  found  only  in  the  Tertiary. 

The  great  petroleum  area  of  the  Eastern  United  States 
is  the  Paleozoic  basin.  In  this  basin  it  is  found  on  sev- 
eral horizons,  l)iit  always  helow  the  coal-measures.  The 
most  celebrated,  viz.,  the  Pennsylvania  horizon,  is  in  the 
Upper  Devonian.  The  Canada  horizon  is  in  the  lowest 
Devonian.     In  West  Virginia  it  is  in  the  sub-Carbonifer- 


PALEOZOIC  ROCKS  AND  ERA,  315 

ous.  In  Ohio  it  is  in  the  Devonian  and  in  the  Silurian, 
especially  the  latter.  In  California  it  is  in  the  Miocene 
Tertiary  of  the  Coast  Kango. 

Origin  of  Petroleum. — It  is  probable  that  petroleum 
was  formed  by  a  change  of  organic  matter,  somewhat 
similar  to  that  which  makes  coal,  but  from  a  different 
kind  of  organic  matter,  and  under  different  conditions. 
Land-plants,  in  the  presence  of  fresh  water,  form  coal ; 
while  marine  plants,  and  sometimes  lower  animals,  in  the 
presence  of  salt  water,  form  petroleum,  bitumen,  etc. 
It  has  been  observed  that  petroleum  is  often  found  in 
connection  with  salt. 

Origin  of  Varieties. — But,  however  formed  in  the 
first  instance,  there  is  no  doubt  that  the  different  varieties 
or  physical  conditions  are  formed  from  each  other  by  the 
passing  away  of  gaseous  hydrocarbon.  In  this  manner 
light  oil  changes  into  heavy  oil,  and  this  into  bitumen, 
and  finally  into  asphalt.  Thus  there  are  two  series  de- 
rived from  organic  matter,  the  coal  series  and  the  petro- 
leum series.  By  successive  changes,  coal  passes  from 
fat-coals  to  bituminous,  then  semi-bituminous,  then  an- 
thracite, and  finally  graphite  ;  petroleum  from  light  oil 
to  heavy  oil,  then  bitumen,  asphalt,  jet,  and  possibly 
diamond.     But  the  origin  of  diamond  is  uncertain. 

Fauna  of  the  Carboniferous  Age, 

As  already  stated,  we  shall  take  up  the  fauna  of  the 
sub-Carboniferous  and  Carboniferous  together ;  only  let 
it  be  remembered  that  the  land  and  fresh-water  animals 
are  from  the  coal-measures,  especially  the  vertebrates, 
and  the  marine  animals  are  mostly  from  the  sub-Car- 
boniferous. We  shall  touch  only  the  most  prominent 
points. 

We  have  nothing  characteristic  to  add  about  corals, 
but  only  draw  attention  here  to  an  exceedingly  purious 


31G 


HISTORICAL   GEOLOGY. 


and  characteristic  form  of  coral-making  Bryozoan,  called 
from  its  j)erfect  screw-like  form,  Arcldmecles  (Fig.  241). 
T]iis  abundant  and  easily  recognized  form  is 
wholly  characteristic  of  the  sub-Carboniferous 
By  studying  the  diagram  (Fig.  243)  the  main 
facts  regarding  Echinoderms  may  be  easily  re- 
membered. As  before  (page  279),  the  lower 
shaded  part  represents  stemmed,  and  the  up- 
per unshaded,  the  free  forms.  The  Cystids, 
it  is  seen,  are  confined  to  the  Silurian,  the 
Blastids  commence  in  the  Silurian,  continue 
through  the  Devonian  and  Carboniferous,  and 
perish  ;  while  the  Crinids  continue  until  now. 
The  Asteroids  commence  in  the  Lower  Silu- 
rian, the  Echinoids  in  the  Carboniferous, 
and  both  continue  until  now — the  species,  of 
course,  changing.  As  Blastids  are  very  abun- 
dant in  the  sub-Carboniferous,  we  give  a  fig- 
ure (Fig.  242). 

Concerning  MoUusca,  we  touch  two  points  : 

1.  .Fresh- water  and  land  shells,  which  were  in- 

FiG.    241.—   troduced  in  the  Devonian,  are  more  abundant 

Archimedes    ^^igs.  244,  245).     2.  The  Goniatites,  first  in- 

Haii.)  troduced   in    the    Devonian,   are    also   more 

numerous  here  (Fig.  246). 
Concerning    Crustacea,    also   two   points : 
1.  While    Trilobites    continue    under    new 
forms,   ready  to  perish  at   the  end  of   this 
period,    Lmiuloids,    or    horseshoe    crabs,    a 
higher  type,  are  here  introduced  (Fig.  247). 
The  transition  from  Trilobites  to  Limuloids 
may  be  quite  perfectly  traced.     2.   True  typi- 
cal crustaceans  of  the  long-tailed  kind  (Ma-   p^,  ais.-Bias- 
crourans),  such  as  shrimps  and  the  like,  were      tid :    Pentre- 
first  introduced  here  (Fig.  248).  '^^  P^^Jf;; 

Insects,  v^hicli  first  appeared  in  the  Devo-      Haii.) 


PALEOZOIC  HOCKS  AND  ERA. 


31' 


aian  in  connection  with  land  vegetation,  as  might  be  ex- 
pected, are  much  more  abundant  and  in  greater  variety  ir 


S  ILURIAN  I      OCV' 


STEM- 
'MED. 


I     N    O   I  D  S 


iiip 

-rZ    c  .   „;h|: 


FREE 


Fig.  244.— Dawsonella  MeekiL  Fig.  245.— Anthracopupa  Ohioensla 

(After  Bradley.)  (After  Whitfield.) 


Fig.  246.  —  Ciirboniferoos 
goniatites :  Goniatites 
crenistria  (European) ;  a, 
side-view  ;  ft,  end-view. 


Pio.   247.— Carboniferous  cmstacean  :    En 
proOps  Danae.    (After  Meek  and  Worthen.) 


the  coal-measures.  There  are  spiders,  scorpions,  centipeds, 
cockroaches,  dragon-flies,  and  beetles  (Figs.  249,  250).  It 
is  well  to  observe   that  the   highest  orders  of  insects, 


318 


HISTORICAL  GEOLOGY. 


flower-loving',  honey-loving,  and  social,  such  as  flies,  hut- 
terflies,  bees,  and  ants  (Dipters,  Lepidopters,  and  HyvM- 


Fig.  248.— Carboniferous  crustacean: 
Anthrapalaemon  gracilis.  (After 
Meek  and  Worthen.) 


Pig.  249. — Carboniferone  insect : 
Blatta  maderae,  wing  -  cases. 
CAf  ter  Lesquereux.) 


nopters),  aro  not  yet 
found,  because  there  are 
not  yet  any  flowering 
plants. 

Fishes. — "We  have 
little  to  add  here  to 
what  has  already  been 
said  under  the  Devo- 
nian. The  same  kinds 
of  fishes — viz..  Ganoids 
and  Placoids — still  pre- 
vail. Of  the  Ganoids, 
however,  the  Placo- 
derms  passed  away  with  the  Devonian,  but  the  Lepido- 
ganoids  continue,  and  some  of  them  become  still  more 
reptilian.  We  note  also  an  advance  in  the  Placoids,  in 
that  an  intermediate  form  (the  Hybodonts)  between  the 
Cestracionts  and  true  sharks  {Squalodonts)  here  appears. 


Fig.  250. — Carboniferous  insects :  Zylobius 
eigillariae.  (After  Dawson.)  a,  anterior ;  b, 
posterior  portion,  enlarged. 


I 


PALEOZOIC  ROCKS  AND  ERA.  319 

Amphibians. — The  introduction  of  amphibians  must 
be  regarded  as  a  great  step  in  the  progress  of  life ;  for 
they  are  the  first  true  land  vertebrates  and  air-breathing 
vertebrates.  Yet  we  must  remember,  on  the  one  hand, 
that  amphibians,  as  their  name  implies,  all  of  them  at 
some  period  of  their  life,  some  of  them  permanently, 
breathe  both  air  and  .water — both  by  gills  and  by  lungs  ; 
and  on  the  other  hand  we  must  remember  that  Ganoid 
fishes  also  supplement  their  gill-breathing  by  lung- 
breathing.  The  amphibians  are  intermediate  between 
fishes  and  true  reptiles.  They  are  represented  now  by 
frogs,  toads,  newts,  etc. 

Now,  in  the  Carboniferous,  and  long  afterward,  am- 
phibians were  very  different  from  any  of  those  mentioned 
as  still  living.  They  belonged  to  a  peculiar  order  now 
long  extinct,  called  Laiyrinthodonts,  from  the  labyrin- 
thine structure  of  their  teeth  (Fig.  252).  All  the  living 
amphibians  are  small  creatures  ;  these  were  often  of  huge 
size.  All  the  living  kinds  have  soft,  moist  skin  ;  these 
were  partly  covered  with 
large,  ganoidal  plates. 
The  early  Ganoids,  too, 
had  the  same  labyrin- 
thine structure  of  the 
teeth,  though  less 
marked  (Fig.  251).  In 
fact,  the  transition  from 

the  reptilian  Ganoids  to  the  ganoidlike  amphibians  of 
the  coal-measures  is  so  gradual  that  it  is  difficult  in  some 
cases  to  say  whether  some  of  these  are  amphibian  reptile 
or  ganoid  fish  (Fig.  253). 

Amphibians  seem  to  have  been  very  abundant  in  the 
coal-measures — some  snakelike  forms,  with  very  small 
or  no  feet,  some  lizardlike  forms,  some  fishlike  forms, 
and  some  huge  crocodilian  forms,  but  not  with  croco- 
dilian affinities.     These  huge  forms  were,  however,  more 


Fig,  251,— Structure  of  a  ganoid  tooth,  (Aftei 

) 


320 


HISTORICAL  GEOLOGY, 


common  later,  i.  e.,  in  the  Triassic.     We  will  describe 

only  two  examples  : 

The  Archegosaurus  {primeval  saurian)   (Fig.   253) 

was  an  animal  two  to  three  feet  long,  with  head  and  body 

much  like  a  ganoid  fish,  and 
covered  with  ganoid  plates  and 
scales.  It  had  probably  per- 
manent gills  as  well  as  limgs, 
and  its  legs  were  little  more 
than  logged  fins,  such  as  are 
found  in  some  ganoids,  and 
wholly  unadapted  for  locomo- 
tion on  land.  It  was  a  re- 
markable connecting  link  be- 
tween ganoid  fish  and  laby- 
rinthodont  amphibian. 

The  Dendrerpeton  {tree- 
reptile)  was  so  called  because 
first  discovered  (by  Dawson) 
in  the  hollow  stump  of  a  sigil- 

laria  tree.     It  was  of  lizard-like  form,  and  about  two  feet 

long      It  is  a  curious  fact  that  these  hollow  stumps  of 


Fig.  262.— Section  of  portion  of 
tootli  of  a  labyrintliodont. 


Fig.  253.— Archegosaurus  :  A,  plates  ;  B,  section  of  tooth. 


sigillaria  filled  with  sandstone  (Fig.  254)  are  very  rich 
in  fossils,  e.  g.,  skeletons  of  amphibians,  remains  of  in- 
sects, and  shells  of  land  mollusca.  The  sigillaria  tree  was 
very  soft  and  spongy,  but  was  covered  with  a  hard  bark. 
We  may  easily  picture  to  ourselves  the  conditions  under 


PALEOZOIC  ROCKS  AND  ERA. 


321 


which  these  remains  were  entombed.  Imagine,  then,  a 
large  sigillaria  tree  on  the  borders  of  a  coal-swamp,  rotted 
down  to  a  hollow  stump.  A  flood  then  carried  thither 
.  floating  insects,  shells,  and  carcasses  of  amphibians,  which 
lodged  in  the  hollow  stump,  and  were  covered  up  with 
sand.  The  hollow  stump 
changed  to  coal,  the  sand  to 
sandstone,  and  the  animal  re- 
mains to  fossils. 

The  very  earliest  amphibian 
known  is  recognized  by  its 
tracks  (Fig.  255).  These  are 
found  in  the  sub-Carboniferous  of  Pennsylvania,  in  a  sand- 
stone marked  with  ripple-marks.  The  animal  has  been 
called  Sauropus  primcBvus  (primeval  reptile-foot).  It 
was  evidently  a  large  Labyrinthodont.  Not  only  tracks 
and  ripple-marks,  but  also  rain-prints  (Fig.  256)  and  sun- 
cracks,  are  common  in  the  coal-measures. 


Fig.  254.— Section  of  hollow  sigil- 
laria stump,  filled  with  sandstone 
(After  Dawson.) 


Fig.  255. —  Slab  of  sandstone  with  sun-cracks  a,  and  reptilian  footprints  6,  from 
coal-measures  of  Pennsylvania  ;  x  \. 


Some  General  Observations  on  the  Whole  Paleo- 
zoic.—  Before  leaving  this  long  and  diversified  era,  we 
must  look  back  and  make  some  general  observations. 

Prog-ressive  Chang-e. — During  the  whole  time  we 
may  observe  a  progressive  change  going  on  :  1.  There 
was,  as  we  have  seen,  a  steady  growth  of  the  continent 

Lk  Contk.  Geol.  21 


322 


HISTORICAL   OEOLOOY. 


from  the  Archaean  nucleus.  2.  There  was  also  a  pro- 
gressive change  in  the  constitution  of  the  atmosphere, 
especially  by  removal  of  excess  of  water  and  CO^,  fitting 
it  for  the  introduction  of  higher  animals.  3.  In  connec- 
tion with  these  physical  changes,  there  were  also  progres- 
sive changes  in  life-forms.* 
Appalachian  Revolu- 
tion.— Thus  we  see  a  slow, 
steady,  progressive  change 
during  the  era.  But  now, 
at  tlie  endf  there  occurred 
one  of  those  great  and  rapid 
changes  in  physical  geogra- 
phy and  climate  which  mark 
the  end  of  the  eras,  and 
a  corresponding  sweeping 
change  in  the  forms  of  life. 
The  Appalachian  chain  was 
formed  at  this  time,  and  is 
its  monument,  and  therefore, 
by  American  geologists,  it  is  fitly  called  the  Appalachian 
Revolution.  The  place  of  the  Appalachian  chain  during 
the  Silurian  and  Devonian  eras  was  the  marginal  sea- 
bottom  of  the  great  interior  Paleozoic  Sea,  receiving 
sediments  until  30,000  feet  were  accumulated.  During 
the  Carboniferous  it  was  sometimes  an  inland  sea-bottom, 
sometimes  a  coal-marsh,  and  sometimes,  perhaps,  a  lake, 
but  always  receiving  sediment  until  10,000  more  feet  were 
accumulated.  Now,  at  last,  it  yielded  to  the  ever-increas- 
ing lateral  pressure,  and  was  folded  and  crumbled  with 
all  its  coal-beds,  and  swelled  up  into  a  great  mountain- 
range.  It  has  since  been  sculptured  by  erosion  into  itb 
present  forms. 

We  have  said  that  the  change  of  life-forms  produced 

*  For  a  fuller  account  of  this  important  point,  the  teacher  is  re- 
ferred to  the  larger  work. 


Fig.  256. 


-Fossil  rain -prints  of  the  Coal 
period. 


PALEOZOIC  ROCKS  AND  ERA.  32g 

by  this  revolution  was  sweeping.  When  quiet  and  pros- 
perous times  again  commenced,  in  the  Mesozoic,  we  find 
an  entirely  different  condition  of  things.  It  is  almost 
like  a  new  world.  We  must  not  imagine,  however,  that 
the  change  was  absolutely  sudden.  The  steps  of  change 
here  were  only  more  rapid,  and  the  general  unconformity 
and  loss  of  record  which  occur  here  make  it  seem  sudden. 

Transition  to  the  Mesozoic  Era. — Permian  Period, 

We  have  seen  (page  267)  that  the  Paleozoic  commenced 
after  a  great  revolution.  Now,  the  Paleozoic  was  closed 
also  by  a  similar  revolution.  We  have  called  this  latter 
the  Appalachian  Eevolution,  because  this  range  was 
made  at  that  time  ;  but  it  was  a  time  of  widespread 
oscillations,  and,  therefore,  of  great  changes  in  physical 
geography  and  climate,  marked  by  universal  unconformity 
and  by  sweeping  changes  in  life-forms.  Now,  as  already 
seen  on  page  193,  unconformity  always  means  lost  record 
at  that  place.  Of  the  lost  record  between  the  Archaean 
and  Paleozoic  nothing  has  been  certainly  found,  but  of 
that  between  the  Paleozoic  and  Mesozoic,  certain  leaves 
have  been  recovered.  These  are  brought  together  and 
called  the  Permian.  Some  have  allied  the  Permian  with 
the  Mesozoic  under  the  name  of  Dyas.  Others  have 
allied  it  with  the  Paleozoic.  The  truth  is,  it  ought  to 
be  regarded  as  a  period  of  transition  or  of  revolution 
between  the  two. 

Life  System. — As  might  be  expected  the  organisms 
of  the  Permian  are  mainly  transitional.  Paleozoic  forms 
are  passing  away  and  Mesozoic  forms  are  coming  in.  The 
very  first  reptiles  are  introduced  here,  but  they  had  not 
yet  attained  supremacy. 


CHAPTER  IV. 

MESOZOIC    ERA. — AGE   OF    REPTILES. 

The  Paleozoic  era  was  very  long  and  diversilied.  It 
consisted  of  three  ages — the  age  of  Invertebrates,  the  age 
of  Fishes,  and  the  age  of  Acrogens  and  Amphibians. 
The  Mesozoic  era,  on  the  contrary,  consists  of  but  one 
age — the  age  of  Reptiles.  Never,  in  the  history  of  the 
earth,  were  reptiles  so  abundant,  of  such  size  and  variety, 
or  so  highly  organized,  as  then. 

Characteristics  of  the  Age. — The  characteristics  of 
this  age  are  the  culmination  of  the  class  of  reptiles,  and 
the  class  of  cephalopod  mollusks  among  animals,  and  of 
cycads  among  plants  ;  and  the  first  introduction  of  mam- 
mals and  birds,  and,  in  the  last  part,  of  Teleost  fishes  and 
Dicot ijledonous  trees.  The  most  striking  characteriRtic  is 
the  culmination  of  reptiles,  and  this,  therefore,  gives  it 
Its  name. 

Subdivisions. — The  Mesozoic  era    and  Reptilian  age 

is  divided  into  three  periods 

r  3.  Cretaceous.     — viz.  :  1.    Triassic,  on  ac- 

Mesozoic  rocks.  <  2.  Jurassic.         count  of  its  distinct  three- 

[  1.  Triassic.         fold   division   in    Germany, 

where    first    well     studied. 

2.  Jurassic,  on  account  of  its  splendid  development  in 

the  folded  structure  of  the  Jura   Mountains.     3.  Creta- 

vcous,  on  account  of  the  chalk  of  England  and  France 

being  one  of  its  members. 

These  three  are  very  distinct  periods  in  Europe,  but 
in  America  the  Trias  and  Juras  are  closely  connected 
324 


MESOZOIC  ERA.—AOE  OF  REPTILES.  3^5 

though  very  distinct  from  the  Cretaceous  ;  so  that,  studied 
in  America  alone,  it  would  be  most  natural  to  divide 
the  whole  age  into  two  periods  :  1.  Jura-  Trias ;  and, 
2.   Cretaceous. 

Again,  the  Jura-Trias  is  much  poorer  in  fossils  in  this 
country  than  in  Europe ;  so  that,  if  we  treated  of  American 
strata  alone,  we  should  give  but  a  very  imperfect  picture 
of  the  times.  Therefore,  our  j^lan  will  be  to  give  a  brief 
sketch  of  the  Trias,  and  then  of  the  Juras,  taking  illus- 
trations chiefly  from  foreign  sources,  and  then  a  sketch 
of  the  Jura-Trias  in  America.  The  Cretaceous  can  be 
fully  illustrated  from  American  strata. 

Section  I. — Triassic  Period. 

As  already  stated,  the  lowest  Mesozoic  (Triassic)  is 
always,  or  nearly  always,  unconformable  with  the  Coal. 
The  line  of  break  may  be  between  the  Triassic  and  the 
Permian,  but  more  commonly  between  the  Permian  and 
the  Coal.  But  tlie  fossils  of  the  Triassic  are  always  very 
different  from  those  of  the  Permian.  The  break  in  the 
life  system  is  always  greatest  here.  We  will  neglect  the 
subdivisions,  and  take  up  all  together. 

Life  System. — Although  the  revolution  which  closed 
the  Paleozoic  is  passed,  and  comparative  quiet  again  re- 
stored, yet  it  took  some  time  for  the  old  fullness  of  life 
to  recover  itself.  Mesozoic  life,  therefore,  is  compara- 
tively poor  in  the  Triassic  compared  with  the  Jurassic 
and  Cretaceous.  We  will,  therefore,  touch  very  briefly 
on  Triassic  life. 

The  Cliaiig-e. — The  most  striking  fact  is  the  sweep- 
ing change  in  life-forms.  All  the  old  style  corals  are 
replaced  by  new  style  ;  all  the  armless  crinoids  (Blastids 
and  Cystids),  the  square-shouldered  brachiopods,  the 
orthoceratites  and  trilobites,  the  lepidodendrids,  sigil- 
larids,  and  calamites — in  a  word,  all  that  we  found  most 


a^e 


HtSTOniCAL   GEOLOGY, 


characteristic  of  the  Paleozoic,  are  gone.  They  are  re- 
placed by  other  and  very  different  forms. 

Plants. — As  the  grand  characteristic  of  the  Coal  pe- 
riod was  the  predominance  of  the  vascular  Cryptogams,  so 
that  of  this  period  is  the  predominance  of  the  next  higher 
group  of  plants,  viz.,  Gymnosperms,  i.  e..  Conifers  and 
Cycads,  especially  Cycads  (see  diagram  on  page  259). 
Ferns  and  Equisetae,  however,  still  abounded,  though  of 
different  genera  from  those  of  the  Coal.  But  as  the  pe- 
culiar flora  of  the  Mesozoic  did  not  culminate  until  the 
Jurassic,  we  shall  put  off  illustrations  until  that  time. 

Animals. — Although  Cystids  and  Blastids  disappear 
with  the  Paleozoic,  the  Crinids  are  still  represented  by 
many  beautiful  new  forms,  with  plumose  arms,  which, 
when  expanded,  must  have  presented  a  truly  flower-like 
appearance,  and  their  fossilized  remains  are  therefore 
often  called  stone-lilieSo     One  of  these  is  shown  in  Fig. 


Fig.  257.— Encrinus  liliformis. 


Fig.  258.— Ceratites  nodosus. 


257,  and  on  page  331  we  give  a  similar  form  in  expanded 
condition. 

The  Goniatites    have   passed  away.      The   Ammonite 


31  E so  ZOIC  ERA.— AGE  OF  REPTILES. 


327 


family  is  here  represented  by  Ceratites.  They  are  easily 
recognized,  and  entirely  characteristic  of  the  Triassic.  The 
complexity  of  the  suture  is  increased,  as  shown  in  Fig.  258. 


Fig.    259.— Teuth      of      Triassic  Fig 

fishes  ;  Hybodus  apicalis.    (Af- 
ter Agassiz.) 


-Mastodonsaurus  Jaegeri 


Among  fishes  we  find  still  only  Ganoids  and  Placoids, 
but  the  Ganoids  are  assuming  more  and  more  the  form  of 
ordinary  fishes  (Teleosts),  and  the  teeth  of  the  Placoids 
are  becoming  more  shark-like  (Fig.  259). 


Fig.  261.— Triassic  reptiles  (after  Owen)— anomodonts  and  therodonts  ;  Dicynodon 
lacerticeps. 


'328 


EISTORICAL   GEOLOGY. 


Amphibians. — Labyrinthodonts,  introduced  in  the 
Coal,  continue  and  culminate  here  (Fig.  260),  and  soon 
become  extinct. 

Reptiles. — Reptiles  were  introduced  in  the  Permian 
but  did  not  become  dominant  until  the  Mesozoic.  Cer- 
tain forms,  of  which  we  shall  speak  hereafter,  commence 
here,  but  culminate  in  the  Jurassic.  But  there  are  also 
some  curious  transitional  forms  entirely  characteristic 
of  this  period.  The  Anomodonts  (lawless-toothed)  were 
beaked  like  a  turtle,   and  either  toothless  or  else  with 

long  tusks  only  (Fig. 
261),  but  crocodilian  in 
form.  The  Therodonts 
(beast-toothed)  were  so 
called  because  their  teeth 
Avere  in  three  groups, 
corresponding  to  inci- 
sors, canines,  and  molars 
of  mammals  (Fig.  262). 
Both  of  these  curious 
families  had  many  char- 
acters allying  them  with 
the  lowest  mammals,  i.  e.,  Monotremes  (Ornithorhynchus, 
Echidna,  etc.),  now  found  only  in  Australia.  They  have 
been  fitly  called,  by  Cope,  Theromorpha  (beast-like). 
These  beast-like  reptiles  seem  to  have  been  introduced 
first  in  the  Permian. 

Mammals. — If  beast-like  reptiles  are  found  here,  we 
might  naturally  expect  also  the  lower  forms  of  beasts 
themselves.  In  the  upjoermost  Triassic,  both  of  Europe 
and  America,  remains  of  small  marsupial  mammals  have 
indeed  been  found  ;  but  as  only  a  feio  have  been  found, 
and  these  in  the  uppermost  Triassic,  almost  passing  into 
the  Jurassic,  and  as  similar  remains  are  far  more  abun- 
dant in  the  Jurassic,  we  shall  put  off  their  description 
until  that  time. 


Fig.  262. — Lycogaurus. 


MESOZOIC  ERA.-AGE   OF  REPTILES.  329 

No  hirds  have  been  found.  It  may  seem  strange  that 
mammals  should  have  been  introduced  before  birds  ;  but 
we  tind  the  explanation  of  this  in  the  fact  that  birds  are 
a  suh-hranch  of  the  reptilian  branch  of  the  vertebrate 
stem. 

Section  II. — Jurassic  Period. 

Name. — These  strata  and  the  period  they  represent 
are  called  Jurassic,  because  of  their  splendid  develop- 
ment in  the  folded  structure  of  the  Jura  Mountains  (Fig. 
145,  page  241)  and  their  richness  in  fossils  there. 

Rock-System. — In  England  the  Jurassic  has  been 
subdivided  into  the  Lias,  the  Oolite,  and  the  Wealden ; 
but  we  shall  neglect  these,  and  speak  only  of  the  whole 
together. 

Coal.-;-One  point  worthy  of  note  here  is  the  occurrence 
of  coal.  The  Jurassic  coal-fields  are  far  smaller  than 
those  of  the  Carboniferous,  but  the  mode  of  occurrence 
of  the  coal  is  much  the  same. 

Examples  of  such  coal  are  the  Yorkshire  and  Brora 
coal  of  Great  Britain,  and  some  of  the  coals  of  India  and 
China  ;  also  the  coals  of  eastern  Virginia  and  North  Caro- 
lina. Of  these  last  we  shall  speak  again.  Many  Jurassic 
coals  are  of  excellent  quality,  though  the  average  is 
inferior  to  the  coal  of  the  Carboniferous. 

Plants. — The  characteristic  families  of  the  Jurassic 
are  Ferns,  Conifers,  and  Cycads.  Conifers  and  Cycads, 
especially  Cycads,  culminated  in  this  period  ;  they  are 
found  in  extreme  abundance  in  connection  with  the 
Jurassic  coal  in  the  form  of  leaves,  trunks,  and  roots. 
Some  Jurassic  plants  and  their  living  allies  are  shown  in 
Figs.  263-266. 

Animals. 

The  culmination  of  the  characteristic  animals  of  the 
Mesozoic,   especially  reptiles,    occurred    in    this   middle 


330 


HISTORICAL  GEOLOGY. 


period.     We  shall  touch  very  briefly  all  except  the  most 
important  characteristic  kinds. 


Fig.  263.— Zaniia  spirali!?,  a  living 
cycad  of  Australia. 


Fig.  264.— Stem  of  cycadeoidca 
megalophylla. 


Crinoids,  beautiful,  plumose-armed,  and  lilylike,  are 
abundant  (Fig.  267)  ;  but  so,  also,  are  the  free  asteroids 
and  echinoids  (Fig.  268).  The  two  kinds,  stemmed  and 
free,  are  evenly  balanced. 

Bivalves  are,  of  course,  abundant  and  of  characteristic 
forms,  in  this  as  in  all  geologi- 
cal times ;  but  we  can  only 
draw  special  attention  to  the 
oyster  family  (including  Os- 
trea,  GrypJiea,  Trigonia,  etc.), 
which  were  first  introduced 
here  (Figs.  269-271). 

Animonites.  —  The  Ammo- 
nite family  were  introduced  first 
in  the  Devonian  as  Goniatites. 
These  were  replaced  in  the  Tri- 
assic  by  Ceratites.  The  Am- 
monites proper,  the  highest 
type  of  the  family,  were  intro- 
duced in  the  early  Mesozoic,  culminated  here  in  the  Juras- 
sic, continued  through  the  Cretaceous,  and  died  out  at  its 


Fig.  265.— Jurassic    plants  :   Ptero 
phyllum  comptum  (a  cycad). 


p 


MESOZOIC  ERA.— AGE  OF  REPTILES. 


331 


end.     It  is,  therefore,  cliaracteristic  of  the  Mesozoic.     In 
the  Jurassic  they  were  of  extreme  abundance,  and  of  all 


Flu.  JiCG.— Jurassic  plants — 
Conifers  :  Cone  of  a  pine. 


Fig.  268.— Clypeus  PlotlL 


Pig.  267.— Apiocriniles  restored. 
(After  Buckland.) 


Fig.  269  Fig.  270.  Fig.  271. 

Figs,  269-271.— Jurassic  lamellibranchp  of  England  :  269.  Trigonia  clavellata. 
270.  Ostrea  Sowerbyi.    271.  Ostrea  Marshii. 


332 


HISTORICAL   GEOLOGY. 


sizes,  from  half  an  inch  to  three  feet  in  diameter.  We 
give  some  ligiires  of  the  most  characteristic  forms  (Figs. 
272-274). 

It  is  interesting  to  trace  tlie  gradual  changes  in  the 


Fig.  272.  Fig.  273.  Fig.  274. 

Figs.  272-274.— Jurassic  cephalopoda— Ammonites  :  272.  Ammonites  margaritanus. 
273.  Ammonites  Jason  :  side-view.  274,  Ammonites  cordatus  :  a,  side-view  -, 
6,  sliowing  suture. 

form  of  the  suture  in  shelled  cephalopods.  In  the  Silu- 
rian Orthoceratites  the  sutures  were  even;  in  the  Devonian 
and  Carboniferous  Goniatites  they  were  angled ;  in  the 


Figs. 


Fig.  276. 
rs,  276. — 275.  Belemnites  Owenii.    276.  Belemnites  unicanaliculatus. 


Triassic  Ceratites  they  were  scalloped  ;  finally,  here  in  the 
Ammonites  they  were  frilled  in  the  most  complex  patterns. 
Belemnites. — Now,  for  the  first  time,  we  find  the 
highest  order  of  cephalopods,  viz.,  the  naked  ones,  allied 
to  the  squids,  cuttle-fishes,  etc.  This  order  is  represented 
in  Jurassic  times  by  a  peculiar  form,  called  Belemnites, 


ME  so  ZOIC  ERA.— AGE  OF  REPTILES. 


333 


from  the  curious,  dartUke  bone  (Figs.  275,  276),  which  is 
often  the  only  part  found.  Sometimes  the  soft  parts  have 
been  found  ;  the  ink-bag  (Fig.  277)  has  been  found  so  per- 
fect that  good  ink  has  been  made  of  it,  and  the  animal 
has  even  been  drawn  with  its  own  fossil  ink.  From  the 
various  parts  found  it  is  possible  to  restore  the  animal  with 
some  confidence.  In 
Fig.  278  we  give  such 
a  restoration,  and  in 
Fig.  279  aliving  squid 
for  comparison. 

Crustaceans  and 
Insects. — There  is  a 
steady  development. 


Fig.  277.  Fig.  278.  Fig.  279. 

Figs.  277-279.-277.  Fossil  ink-bags  of  Belemnites.    278.  Belemnite  restored. 
279.  A  living  squid. 


(luring  the  Mesozoic,  of  crustaceans,  toward  the  highest 
form,  viz.,  the  crabs.  This,  however,  was /6?tV??/ attained 
only  in  the  Cretaceous,  though  a  spider-crab  has  been 
found  in  the  Jurassic. 

Insects  also  are  far  more  numerous  and  diversified  (Figs. 
280,  281)  than  heretofore,  although  even  yet  the  highest 
forms,  such  as  ants,  bees,  and  butterflies,  are  not  found. 

There  is  little  of  importance  to  be  noted  in  regard  to 
Fishes.     We  therefore  pass  on  to  Reptiles. 


334 


HISTORICAL  GEOLOGY. 


Fig.  280,— Jurassic  in- 
sects :   Blattina  for- 
(AfterHeer.) 


Reptiles.— These  are  the  rulers  of  the  age,  and  cnl- 
minate  in  this  period.  We  shall  therefore  dwell  a  little 
more  fully  on  them.  During  the  Jurassic  there  was  a  truly 
extraordinary  development  of  this  class,  in  number,  size, 
variety,  and  degree  of  organization.  They  were  rulers  in 
every  department  of  Nature :  rulers  in 
the  sea,  in  place  of  whales  and  sharks  of 
to-day;  rulers  on  the  land,  in  place  of 
beasts ;  and  rulers  in  the  air,  in  place 
of  birds.  We  shall  take  them  up  under 
the  three  heads  indicated,  viz.:  1.  Ma- 
rine Saurians  (Enaliosaurs).  2.  Land 
Saurians  (Dinosaurs).  3.  Winged  Sau- 
rians (Pterosaurs).  The  first  were  swim- 
ming, the  second  walking,  the  third  fly- 
ing, animals. 

1.  Marine  Saurians. — Among  these 
I      If  we  shall  mention  only  the  two  most  noted, 

viz..  Ichthyosaurus  and  Plesiosaurus. 
The  Ichthyosaurus  (fish-reptile)  (Fig. 
282)  was  a  huge  monster,  thirty  to  forty 
feet  long,  with  thick  body,  short  neck, 
enormous  head,  eyes  twelve  to  fifteen 
inches  in  diameter,  and  Jaws  set  with 
hundreds  of  conical  teeth.  The  limbs 
were  paddles,  suitable  for  swimming,  not  for  walking. 
The  powerful  tail  was  expanded  vertically  into  a  fin  at  its 
extremity,  and  the  bodies  of  the  vertebrae  were  biconcave 
like  those  of  a  fish.  Perfect  skeletons  of  this  animal 
have  been  found ;  and  even  the  impressions  of  its  intes- 
tines, and  the  contents  of  its  stomach,  revealing  the 
nature  of  its  last  meal,  have  been  preserved. 

The  Plesiosaurus  (lizardlike)  (Fig.  283)  was  a  slenderer 
animal,  with  a  very  long  neck,  small  head,  short  tail,  long 
and  powerful  paddles,  and  fishlike  vertebrae. 

2.  Dinosaurs,   or   Land    Saurians. — The   hugest   of 


Fig.  281.— Glaphyrop- 
tera  gracilis,  (After 
Heer.) 


MESOZOIC  ERA.— AGE  OF  REPTILES  335 

reptiles — in  fact,  the  iiugest  animals  which  have  ever 
walked  the  earth — were  of  this  order.  They  Avere  also 
the  most  highly  organized  ot  reptiles  ;  for,  if  the  marine 
aaurians  connected  this  class  with  fishes,  the  dinosaurs 


Pig.  282.--JuraBBic  reptiles— Ichthyosaurus  and  Plesiosauius  :    chthyosaurus  com- 
munis, X  1^5. 

connected  it  with  the  higher  class  of  hirds.  Some  of  the 
characters  connecting  them  with  birds  are  the  following: 
1.  Many  of  them  had  long,  powerful  hind-legs,  large  hip- 
bones, and  strong  sacrum,  and  very  short  and  small  fore- 


Pig    283.— Jurassic  reptiles— Iclithyosaurus  and  Plesiosaurus  :  Plesiosaurus  doli- 
chodeirus,  restored,  x  5*5. 

legs.  These  characters  show  that  they  walked  mainly  on 
their  hind-legs,  in  the  manner  of  birds.  2.  Many  of  them, 
like  some  birds,  had  only  three  toes  on  the  hind-feet,  so 
that  they  made  tracks  which  were  bird-like.  3.  There 
were  peculiarities  about  their  ankle-joints  which  were 
still  more  bird-like. 


336 


BlSTORlCAL   OEOLOQY, 


Pig.  284. — Iguanodon  Bemissartensis.    (After  Marsh.) 


The  most  noted  of  this  order  found  in  Europe  are  the 
Iguanodon  and  the  Megalosaurus. 
The  iguanodon  (Iguana-tooth), 
judging  from  the  size  of  its  bones, 
was  probably  several  times  more 
bulky  than  the  elephant ;  and  yet 
a  perfect  skeleton,  recently  found 
in  Belgium  (Fig.  284),  shows  that 
it  walked  on  the  hind-legs  alone, 
supporting  itself  by  its  massive  tail. 
The  neck  was  long,  flexible,  and 
bird-like,  and  the  jaws  were  beaked 
in  front  and  set  with  herbivorous, 
iguanalike  teeth  (Fig.  285)  behind. 
The  megalosaur  (great  saurian) 
was  not  quite  so  large,  but  prob- 
ably still  more  formidable,  since  it 
was  carnivorous.  A  restoration  of  a  smaller  allied  form 
is  given  in  Fig.  280.      This  also  walked  mainly  on  two 


Fig.  285. 


-Tooth  of  an  Igua- 
nodon. 


MESOZOIC  ERA.— AGE  OF  REPTILES.  337 

legs.     Still  much  larger  animals  of  this  order  have  been 
found  in  the  United  States,  as  we  shall  see  further  on. 

3.  Pterosaurs,  or  Winged  Sauriaiis. — These  are  per- 
haps the  most  extraordinary  of  all  known  animals.     They 


Fig.  28G.— Compsognatlms,  x  i»jj.    (Restored  by  Marsh.) 

combined  the  stout  body  with  keeled  breastbone,  the  long, 
flexible  neck  and  beaklike  jaws  of  a  bird,  with  the  long 
arms  and  membranous  flying-web  of  a  bat  and  the  essen- 
tial characters  of  a  reptile.  In  some  cases  they  had  a 
short,  aborted  tail,  like  a  bird,  but  in  others  a  long  tail, 
with  vertical  expansion  at  the  tip,  which  was  used  as  a 
rudder  in  flying  (Fig.  287).  The  pterosaurs  varied  in 
size  from  two  or  three  feet  to  eighteen  or  twenty  feet 
from  tip  to  tip  of  the  extended  wings. 

Birds. — We  have  seen  that  the  reptiles  of  this  time 
approached  birds,  but  still  more  remarkably  do  the  earliest 
birds  approach  reptiles.  There  is  in  Bavaria  a  peculiar 
limestone  used  the  world  over  for  lithographic  drawings. 
This  lithographic  limestone  is  equally  celebrated  for  its 
marvelous  preservation  of  fossils.  In  186,2  the  oldest 
known  bird,  the  Archceopteryx,  was  found  there  with  even 

Le  Contk,  Geol,  22 


338  HISTORICAL   GEOLOGY. 

the  feathers,  and  the  minute  structure  of  the  feathers  of 
the  wings  and  tail,  preserved.     An  undoubted  bird,  yet 


Pig.  287.— Restoration  of  Rhamphorhynchus  phylluruB.    (After  Marsh.) 
One  seventh  natural  size. 

how  different  from  modern  birds  !  Instead  of  the  short, 
aborted  tail,  bearing  feathers  radiating  almost  from  one 
point,  as  in  all  modern  birds,  it  had  a  long  reptilian  tail 
with  twenty-one  joints,  and  the  feathers  given  off  in  pairs 
on  the  two  sides  of  each  joint.  Among  many  other  rep- 
tilian characters  are  the  possession  of  socketed  teeth,  and, 
instead  of  the  hand  being  wholly  consolidated  to  form  the 
wing,  as  in  modern  birds,  the  three  fingers  remain  free, 
and  are  armed  with  claws  (Fig.  288). 

Another  fine  specimen  of  this  wonderful  bird  was 
found,  in  1873,  in  the  same  locality,  and  is  now  in  the 
Berlin  Museum.  In  the  Jurassic  dinosaurs  and  this 
Jurassic  bird  we  have  excellent  examples  of  what  we 
have  called  generalized  or  connecting  types.  These  two 
branches — reptile  and  bird — which  seem  so  widely  dis- 
tinct now,  when  traced  backward  in  time,  approach  more 
and  more,  until  we  find  almost  their  point  of  union. 

Mammals. — We  have  already  stated,  on  page  328,  that  a 
few  small  marsupial  mammals  are  found  in  the  uppermost 
Triassic,  both  of  Europe  and  the  United  States.  These 
we  regarded  as  anticipations,  and  therefore  put  off  their 


MESOZOIG  ERA.— AGE  OF  REPTILES. 


339 


discussion.     This  anticipation  is  fully  realized  in  the  Ju- 
rassic.    In  England  there  have  been  found  about  eighteen 


species,  and,  in  the  United  States,  Marsh  has  found  seven- 
teen species;  so  that  there  are  now  known  about  thirty-five 
species  of  Jurassic  and  three  species  of  Triassic  mammals. 


340 


HISTORICAL   GEOLOGY. 


But,  as  the  first  birds  were  not  true  typical  birds,  but 
reptilian  birds,  so  also  the  earliest  mammals  were  not  true 
typical  mammals,  but  reptilian  mammals,  or  marsupials. 
The  marsupials  live  now  almost  wholly  in  Australia.  Tliey 
include  the  kangaroos,  the  opossums,  the  bandicoots,  the 
wombats,  etc.  In  Jurassic  times  they  apparently  inhab- 
ited all  parts  of  the  earth  in  great  numbers.     Now,  the 

marsupials  differ  so 
greatly  from  ordinary 
mammals  that  they  are 
put  into  a  distinct  sub- 
class. One  striking  pe- 
culiarity about  them  is 
that  their  young  are 
born  in  an  exceedingly  imperfect  state,  so  that  they  are 
almost  egg-bearing,  semi-oviparous. 

But  neither  were  the' Jurassic  marsupials  typical  mar- 
supials, but  rather  generalized  types  connecting  with  In- 
sectivora,  the  lowest  of  the  true  mammals.  They  were  all 
small  animals,  varying  in  size  from  that  of  a  mole  to  that 
of  a  skunk.  They  were  not  able  to  contend  for  mastery 
with  the  great  reptiles.  The  reign  of  mammals  had  not 
yet  come.     We  give  here  (Figs.  289,  290)  a  jaw  of  a  Ju- 


FiG.  289.— Jaw  of  a  Jiiiassic  mammal 
Amphitharium  Prevostii. 


Fig.  290.— Myrmecobius  fasciatub,  banded  aut-eater  of  Australia. 


MESOZOIC  ERA.— AGE  OF  REPTILES.  341 

rassic  marsupial,  and  also  a  living  marsupial  most  nearly 
allied  to  them. 

Section  III. — Jura-Trias  iit  America. 

Areas;  Atlantic  Border. — All  along  the  eastern  slope 
of  the  Appalachian  chain,  from  Nova  Scotia  to  South  Caro- 
lina, in  the  Archgean  region  of  the  map  on  page  272,  are 
found  elongated  patches  of  sandstones  and  shales  which 
belong  to  this  period.  One  of  these  is  in  Nova  Scotia 
and  Prince  Edward  Island  ;  the  next,  going  south,  is  the 
celebrated  Connecticut  River  Valley  sandstone  ;  the  next 
a  long,  narrow  patch  commencing  in  New  York,  passing 
through  New  Jersey,  Pennsylvania,  Maryland,  and  ending 
in  northern  Virginia  ;  then  two  or  three  patches  in  eastern 
Virginia,  about  Richmond  and  Piedmont  ;  and,  lastly, 
some  on  the  Deep  River  and  the  Dan  River  of  North 
Carolina.  They  all  lie  in  hollows  unconformably  on  the 
Archaean  gneiss,  and  therefore  their  age  can  not  be  known 
except  by  fossils  ;  but  these,  though  few,  seem  to  indicate 
that  they  represent  the  whole  Jura-Trias,  although  most 
writers  speak  of  them  as  Triassic.  In  all  these  patches 
are  found  remarkable  outbursts  of  igneous  rocks,  often 
columnar  in  structure,  which  by  erosion  have  formed  the 
so-called  trap-ridges.  Such  are  Mounts  Tom  and  Holy- 
oke,  in  the  Connecticut  Valley  patch,  and  the  Palisades 
of  the  Hudson  River  in  the  New  Jersey  patch. 

Interior  Region. — Red  sandstones,  poor  in  fossils, 
but  probably  referable  to  this  period,  are  found  in  many 
places  in  the  Plateau  and  Basin  regions. 

Pacific  Slope. — On  both  sides  of  the  Sierra,  rocks  of 
this  age,  in  a  metamorphic  condition,  form  the  auriferous 
slates  of  this  region. 

Life-System. 

Life,  no  doubt,  abounded,  but  the  conditions  were 
unfavorable  for  preservation.     We  can,  therefore,   take 


343  HISTORICAL  GEOLOGY. 

up  only  a  few  of  these  localities  and  give,  briefly,  the 
findings, 

lo  Connecticut  River  Valleyo — This  celebrated  local- 
ity is  classic  ground,  through  the  life-long  labors  of  Dr. 
Hitchcock.  The  patch  is  one  hundred  and  fifty  miles 
long  and  ten  to  fifteen  miles  wide,  extending  from  New 
Haven  Bay,  on  Long  Island  Sound,  through  Connecticut 
and  Massachusetts,  and  mostly  on  the  two  sides  of  the 
Connecticut  Rivero  As  the  strata  dip  regularly  to  the 
east,  their  thickness  is  easily  estimated,  and  seems  to  be  at 
least  5,000  to  10,000  feet.  They  consist  of  red  sandstones 
and  shales,  and  are  in  some  places  beautifully  fissile.  As 
might  be  expected  from  their  redness,*  they  are  very  poor 
in  fossils  proper  ;  but  in  certain  parts  an  immense  num- 
ber of  tracks  of  various  animals  have  been  found.  There 
are  tracks  of  (a)  insects  and  crustaceans  j  (b)  of  reptiles ; 
{c)  possibly,  but  not  probably,  of  birds. 

(a)  Insects  and  Crustaceans. — Of  the  insect  and 
crustacean  tracks  little  can  be  made  out  with  certainty. 
We  give  an  example  (Fig.  291). 

(b)  Reptiles. — The  reptilian  tracks  vary  in  size,  from 


FiG>  291.~Tracks  of  insects.    (After  Hitchcock.) 

those  of  a  lizard  to  those  of  the  huge  Otozoum,  twenty- 
two  inches  long  with  a  stride  of  four  feet.  In  character, 
some  are  five-toed,  some  four-toed,  some  three-toed ; 
some  walked  on  four  feet,  some  on  only  two  hind-feet ; 
some  had  long,  dragging  tails  (Fig.  292),  and  some  short 
tails,  or  none  at  all  (Figs.  293,  294). 

(c)  As  already  said,   some  of  these  reptiles  walked  on 
two  legs  only,  and  had  only  three  functional  toes,   and 
*  Organic  matter  decolorizes  sandstones. — See  page  89. 


MESOZOIC  ERA.— AGE  OF  REPTILES, 


343 


some  were  short-tailed  or  tailless.     These  have  been  re- 
garded by  some  as  wingless  birds.     They  were  probably 


C3 


Pig.  292.  Fig.  293.  Fig.  294. 

Figs.  292-294.— Reptile  tracks  (after  Hitchcock) :  293.  Gigantitherium  caadatam, 
X  ^.  293.  Anomoepue  minor*,  x  ^  :  a,  hind-foot ;  6,  fore-foot.  294.  Track  of 
Brontozoum  giganteum,  x  |. 

all  reptiles.  One  of  these  wonderful  two-legged  reptiles 
is  given  in  Fig.  295. 

The  general  conclusion,  then,  is  that  all  these  tracks 
were  those  of  Dinosaurs  and,  possibly,  Labyrinthodonts. 
In  Jura-Trias  times  there  seems  to  have  been  in  this 
place  an  estuary,  into  which  th^  tides  ebbed  and  flowed. 
At  low  tide,  reptiles  of  many  kinds  were  in  the  habit  of 
walking  on  the  soft,  exposed  mud  in  search  of  food  left 
by  the  retreating  tide.  The  incoming  tide  covered  the 
tracks  with  fine  sediment,  and  preserved  them  till  now, 
the  sediments,  meantime,  hardening  into  stone. 

2.  New  Jersey  Patch. — In  this  patch  we  find  the 
same  redness  of  the  sandstone,  and  therefore  the  same 
poverty  of  fossils.     Of  this  sandstone  have  been  built  all 


344 


HISTORICAL  GEOLOGY, 


the  brownstone  houses  of  New  York  city.     A  few  bones 
and  teeth  of  reptiles,  however,  have  been  found,  and  these 


Fig  395.— Anchisaurus  colurus,  x  j*,,  from  Connecticut  sandstone.    (After  Marsh.) 

confirm  the  conclusions  already  given.  A  few  tridactyl 
tracks  also  have  been  recently  found,  similar  to  those  of 
the  Connecticut  patch.  In  Fig.  296  we  give  a  restoration 
of  fish  from  the  New  Jersey  sandstone. 


Fig.  296.— Diplurus  longicaudatns,  x  J.    (After  Dean.) 

3.  Virginia  and  North  Carolina  Patches. — These 
are  very  different  from  the  Northern  patches.  They  form 
the  Richmond  and  Piedmont  coal-fields  of  eastern  Vir- 
ginia (Fig.  297)  and  the  Deep  River  and  Dan  River  coal- 


MESOZOIC  EEA.^AGE  OF  REPTILES. 


345 


Fig  297.— Section  across  Richmond  coal-field.    (After  Daddow.) 


Fig.  298.— Jaw  of  Dromatherium  sylvestre. 


fields  of  North  Carolina.  In  connection  with  the  Coal, 
plants  have  been  found  in  considerable  abundance.  They 
are  those  characteristics 
of  the  Jura-Trias  every- 
where, viz.,  ferns,  cy- 
cads,  and  conifers.  In 
North  Carolina  the  jaw 
of  a  small  marsupial  has 
been  found  about  the  middle  of  the  series  (Fig.  398). 

The  coal  of  these  Jura-Trias  fields  is  of  good  quality, 
in  thick  seams,  and  easily  worked. 

4.  Atlaiitosaur  Beds. — These  we  describe  separately, 
not  only  because  they  are  recent  discoveries,  but  also 
and  chiefly  because  they  belong  to  an  entirely  diiferent 
horizon,  viz.,  the  uppermost  Jurassic  passing  into  the 
Cretaceous. 

In  these  uppermost  Jurassic  beds,  called  Atlantosaur 
beds,  from  their  most  abundant  and  characteristic  genus. 


Fig.  299. — Brontosaurus  excelsis,  x  j^.    (Restored  by  Marsh.) 


have  recently  been  found,  in  Wyoming  and  Colorado, 
great  numbers  of  most  extraordinary  reptiles,  the  largest 
yet  known,  and  also  a  bird  and  seventeen  species  of  small 
marsupial  mammals. 


346 


HISTORICAL  GEOLOGY. 


Reptiles. — The  extraordinary  number  of  dinosaurian 
reptiles  found  here  have  thrown  much  light  on  this  order. 
Some  of  them  were  reptile-footed  {Sauropoda)  (Fig.  299), 
some  bird-footed  (OrnUhopoda)  (Fig.  300),  some  beasts 


Fie.  800.— Laosaurus,  x  ^.    (Restored  by  Marsh.) 


Fig.  301,— Stegosaunisungulatus,  x  i^.    (Restored  by  Marsh.) 


MESOZOIC  ERA.— AGE  OF  REPTILES.  347 

footed  ( Theropoda),.  and  some  curious  plate-covered  rep- 
tiles {Stegosauria)  (Fig  301).  The  Ornitliopoda  and  some 
Theropoda  walked  almost  wholly  on  their  hind-legs  in  the 
manner  of  birds.  The  size  of  some  of  these  reptiles  is 
almost  inconceivable.  A  thigh-bone  of  an  Atlantosaur, 
found  by  Marsh,  was  six  or  seven  feet  long,  and  a  vertebra 
of  an  Amphicoelias,  found  by  Cope,  was  six  feet  high  to 
the  top  of  the  spinous  process.  The  Atlantosaur  has 
been  estimated  to  have  been  seventy  to  eighty  feet  long  ! 
In  the  same  beds,  as  already  stated,  were  found  the 
remains  of  a  bird  and  of  seventeen  species  of  marsupials. 
A  figure  of  one  of  these  is  herewith  given  (Fig.  302). 


Pia.  302.— Right  lower  jaw  of  Diplocynodon  victor  (after  Marsh),  outside 
view— twice  natural  size. 


Disturbances  whicli  closed  the  Jura-Trias  Pe- 
riod.— One  of  the  most  important  changes  which  oc- 
curred at  the  close  of  this  period  was  the  formation  of 
the  Sierra  Nevada  Range.  Until  that  time  the  Pacific 
shore-line  was  east  of  the  Sierra,  and  the  place  of  this 
range  was  a  marginal  sea  bottom  receiving  sediment. 
These  sediments  finally  yielded  at  the  close  of  this  period 
and  were  folded  and  swelled  up  into  this  great  range. 
Subsequent  erosion  sculptured  it  into  its  present  grand 
forms.  Coincidently  with  this  change  in  the  West,  there 
were  on  the  Atlantic  border  outbursts  of  igneous  matter 
forming  the  trap  ridges.     In  the   interior  region  there 


348  HISTORICAL  QEOLOQY, 

was  a  downward  movement  of  the  crust  over  tlie  whole 
Plains  and  Plateau  region  by  which  isolated  inland  seas 
were  changed  into  the  great  interior  Cretaceous  sea. 
The  Sierra  Nevada  Range  is  the  most  conspicuous  monu- 
ment of  this  period  of  change,  and  therefore  it  may  be 
called  the  Sierra  revolution. 

Section-  IV. — Cretaceous  Rocks  and  Period. 

General  Characteristics. — The  Cretaceous  is  in  some 
respects  a  transition  to,  and  a  preparation  for,  the  next 
era.  Mesozoic  types,  such  as  the  great  reptiles,  the  am- 
monites, etc.,  continue,  but  Cenozoic  types,  like  dicoty- 
ledonous trees  and  teleost  fishes,  are  introduced,  and  the 
two  kinds  of  types  coexisted  side  by  side. 

Rock  System  ;  Areas. — 1.  In  the  Atlantic  border 
region,  going  southward,  we  find  no  cretaceans  until  we 
reach  Long  Island.  Going  south  from  this,  we  find  a  strip 
running  through  New  Jersey,  Delaware,  and  Maryland, 
lying  directly  against  the  Archaean ;  then  small,  isolated 
patches  exposed  by  erosion  in  North  Carolina,  South 
Carolina,  and  Georgia.  It  doubtless  extends  all  along 
the  Southern  coast,  but  is  mostly  covered  with  later  Ter- 
tiary deposits.  2.  In  the  Gulf  border  region  it  forms  a 
broad,  crescentic  band,  commencing  in  western  middle 
Georgia,  passing  through  middle  Alabama,  turning  north- 
ward  through  Mississippi  and  Tennessee,  to  near  the 
mouth  of  the  Ohio.  It  underdips  the  Tertiary  of  the 
Mississippi  River  region,  and  reappears  on  its  west  side 
(see  map,  page  272).  3.  It  thence  passes  northward, 
covering  nearly  the  whole  Plains  and  Plateau  region^ 
though  largely  concealed  by  the  Tertiary.  4.  On  the 
Pacific  border  it  is  found  on  the  lower  foot-hills  of  the 
Sierra  Nevada  in  Northern  California,  and,  together  with 
the  Tertiary,  forming  the  whole  of  the  Coast  Range. 

Physical  Geog-raphy. — I^Vom  this  distribution  we  can 


I 


3IES0Z0IC  ERA.— AGE  OF  REPTILES. 


349 


make  out  with  some  confidence  the  condition  of  the  con- 
tinent in  Cretaceous  times.  1.  North  of  New  York  the 
Atlantic  shore-line  was  farther  out  than  now.  It  crossed 
the  present  shore-line  near  New  York,  passed  along  the 
inner  border  of  the  Cretaceous  of  New  Jersey,  Delaware, 
and  Maryland,  and  southward  nearly  along  the  limit  of 
the  low  countries.     2.  The  Gulf  shore-line  went  through 


Fig.  303.— Map  of  North  America  in  Cretaceous  times. 

middle  Alabama,  and  northward  to  the  mouth  of  the 
Ohio,  and  southward  again  on  the  other  side  of  the  Mis- 
sissippi River.  3.  Connected  Avith  this  extended  gulf 
was  a  great  inland  sea  five  to  six  hundred  miles  wide, 
covering  the  whole  Plains  and  Plateau  region  (with  some 
islands  in  the  Colorado  mountains  region),  and  stretching 
northward  probably  even  to  the  Arctic  Ocean,  and  thus 
dividing  tlie  continent  into  two  parts,  an  Eastern  or 
Appalachian  continent  and  a  Western  or  Basin  regior 


350  HISTORICAL  GEOLOGY. 

continent.  The  place  of  the  Wahsatch  Range  wa.a  cheu 
the  western  marginal  bottom  of  this  interior  sea.  4.  The 
Pacific  shore-line  was  then  east  of  the  Coast  Ranges,  and 
its  waves  beat  against  the  lowest  foot-hills  of  the  Sierra. 
This  is  shown  in  the  map.  Fig.  303. 

Character  of  the  Rocks. — In  regard  to  the  kind  of 
strata,  there  are  two  points  worthy  of  passing  mention. 

1.  Chalk. — The  period  takes  its  name  from  the  chalk 
of  England  and  France,  which  belongs  here.  ChalJc  is 
a  soft,  snow-white,  very  pure  lime-carbonate,  scattered 


Fig  304  —View  of  Iowa  chalk  under  the  microscope.    (After  Calvin.) 


through  which  are  nodules  of  flint.  On  account  of  its 
softness,  it  is  worn  into  strange,  castellated  forms.  Pure 
chalk,  as  described,  was  until  recently,  supposed  to  be 
confined  to  England,  and  France,  and  middle  Europe,  bu' 
has  now  been  found  in  the  Cretaceous  of  Texas  and  the 
Plains.     When  examined  with  the  microscope,  it  seems 


ME  so  ZOIC  ERA,~AOE  OF  REPTILES. 


351 


to  be  composed  wholly  of  the  remains  of  low  organisms, 
chiefly  foraminifera  (Fig.  304).  The  flints  are  seen  to  be 
composed  of  shells  of  Diatoms  and  spicules  of  sponges. 
Now,  as  already  shown  (page  117),  this  is  exactly  the 
composition  of  deep-sea  ooze  (globigerina  ooze),  except 
that  the  silica  has  been  separated  and  collected  in  nod- 
ules. It  seems  probable,  therefore,  that  chalk  is  a  deep- 
sea  ooze  of  the  Cretaceous  times. 

2.  Coal. — Coal  is  found,  again,  in  the  Cretaceous,  both 
in  the  United  States  and  elsewhere.  But  as  most  of  our 
later  coal  belongs  to  a  transition  period  between  the  Cre- 
taceous and  the  Tertiary,  we  shall  put  off  the  discussion 
of  these  for  the  present. 

Life- Sy stem  ;  Plants. 

So  great  is  the  change  and  the  advance  in  plants  at 
this  point,  that  if  we  were  guided  by  plants  alone,  we  would 
say  that  the  Cenozoic  era  commenced  with  the  Cretaceous. 
Here  the  present  aspect  of  field  and  forest  seems  to  begin, 
for  here  were  introduced  for  the  first  time,  and  in  great 
numbers,  dicotyls,  or  ordinary  liard-ioood  trees.  The  sud- 
denness of  their  appearance,  however,  is  due,  in  part  at 


Fig.  305.  Fig.  306.  Fig.  307. 

Pigs.  305-307.— Cretaceous  plants  (after  Lesquereux):  305.  Sassafras  araliopsifi 
SO*).  Salix  proteaefolia.    307.  Pagus  polyclada.    All  reduced. 


352 


HISTORICAL   GEOLOGY. 


least,  to  a  lost  interval  between  the  Jura-Trias  and  the 
Cretaceous.  Of  the  four  hundred  and  sixty  species  of 
plants  found  in  the  Middle  Cretaceous  of  the  West,  four 
hundred  are  dicotyls.  Nearly  all  the  genera  of  common 
trees  are  represented,  although,  of  course,  the  species  are 
extinct.  There  were  then,  as  now,  oaks,  maples,  willows, 
sassafras,  dogwoods,  hickory,  beech,  poplar,  tulip-tree 
{Liriodendron),  walnut,  sycamore,  sweet-gum  {Liqvid- 
amhar^,  laurels,  myrtles,  etc.  A  few  of  these  are  given 
in  Figs.  305-307. 

Animals, 

Protozoa. — Though  these  are  found  in  nearly  all  the 
strata  lieretofore  described,   avc  have  usually  neglected 


Fig.  308.  Fig.  309.  Fig.  310. 

Figs.  308-310.— Foraminifera  of  chalk,  magnified :    308.  Flabellina  rugosa.     309. 
Lituola  nautiloides.    310.  Chrysalidina  gradata.    (After  D'Orbigny.) 

chem,  because  they  are  inconspicuous.     But  here  in  the 

Cretaceous  they 
are  so  abundant 
that  they  demand 
attention.  Chalk, 
as  already  said,  is 
almost  w  li  o  1 1  y 
made  up  of  for- 
aminifers  (Figs. 
308-310),  and 
sponges  are  also 
Of  the  former,  some  are  identical 


Fig.  311.— Echinoids  of  the  Cretaceous  of  Europe 
Galerites  albogalerus. 


extremely  abundant, 
with  living  species. 


MESOZOIC  ERA.— AGE  OF  REPTILES. 


353 


Fig.  312. 


-Hippurites  Toucasiana,  a  large  individual  with  two  small 
(After  D'Orbigny.) 


ones  attached. 


Echinoderms  are  now  almost  wholly  of  free  forms. 
The  highest  echinoids  are  especially  abundant.  And, 
what  is  remarkable, 
those  from  the  chalk 
are  very  like  those 
still  living  in  deep 
seas.  The  reason  of 
this  is  that  deep-sea 
conditions,  and  there- 
fore species,  change 
far  more  slowly  than 
those  of  shallow  water 
and  land. 

Bivalve  Shells. — 
Among  the  immense 
number  of  bivalve 
species  found  here, 
we  mention  only 
the  oyster  family,  of 
which  there  are  many 
species,  and  the 
strange  Hippurite 
family  (Fig.  312).  Surely  no  one,  from  its  general  form, 
would  imagine  that  these  latter  were  bivalves. 

Ceplialopofls. — The  Ammonites  and  Belemnites  still 

Lb  Conte,  Gkol.  23 


Fig.  316. 


Fig.  315. 

Figs.  313-316.— 313.  Toxoceras  annulare.  314. 
Hamites  attenuatus.  315.  Ancyloceras  epinige- 
rum.  316.  Baculites  anceps,  x  J.  (After  Wood- 
ward.) 


354 


HISTORICAL   GEOLOGY, 


continu  s  in  great  numbers,  thongh  they  disappear  at  the 
end  of  the  Cretaceous  ;  but,  in  addition  to  the  usual  form, 
the  Ammonites  take  on  now  the  most  strange  and  un- 
accountable shapes  (Figs.  313-316).  Some  are  partly 
uncoiled,  as  in  Scapliites  (boat),  Toxoceras  (bow-horn), 
A ncyloceras  (curved-horn),  Ilamites  (hook)  ;  in  some, 
completely  uncoiled  and  straight,  as  Baeulites  (staif). 
Sometimes  they  are  coiled  spirally,  like  a  gasteropod,  as 


Pig.  817.— Cretaceous  fishes— Teleosts  :  Osmeroides  Mantelli. 


in  Turrulites.  But  for  the  complexity  of  the  suture,  no 
one  would  imagine  a  baculite  or  a  turrulite  to  belong  to  the 
Ammonite  family.  It  is  probable 
that  rapidly  changing  and  unfav- 
orable conditions  tend  to  produce 
new  and  strange  forms.  The  Am- 
monite family  were  on  the  point 
of  becoming  extinct. 

Fishes. — Here  we  note  another 
great  step  in  the  progress  of  life. 
The  Teleost  fishes,  the  vastly  pre- 
dominant kind  at  the  present  day, 
are  here  first  introduced,  and  al- 
most immediately  become  abun- 
dant.    The   Ganoids  at  once  be- 
come   very     subordinate.       The 
sharks,  however,  are  abundant  and  of  large  size,  and  of  the 
highest  kind,  viz.,  Squalodonts,  or  true  sharks  (Fig.  318). 
Reptiles. — If,  in  Europe,  reptiles  seem  to  have  culmi- 


)h.G .    31 8  —  Cretaceous 
Sharks:  Otodus.  (Af ter Leidy.) 


MESOZOIG  ERA.— AGE  OF  REPTILES. 


355 


nated  in  the  Jurassic,  in  America  they  seem  to  have  cul- 
minated in  the  uppermost  Jurassic  and  Cretaceous.  The 
great  interior  Cretaceous  sea  and  adjacent  land  seems  to 
have  swarmed  with  marine  and  land  reptiles  of  incredible 
size.  All  the  kinds  already  spoken  of  under  the  Jurassic 
were  found  also  in  the  Cretaceous,  and  in  addition,  one 
order,  the  Mosasaurs — wholly 
characteristic  of  the  Cretace- 
ous. The  accompanying  sched- 
ule will  give  some  idea  of  the 
number  of  species,  and  the 
kinds,  of  these  reptiles.  We 
shall  not  again  describe  most 
of  these,  but  only  mention  a 
few  interesting  points  : 

The  mar  me    saurians  were 
represented  in  America  only  by  the  long-necked  kinds 
(Flesiosaurs) ;  but  these  were  numerous,  and  some  greatly 
surpassed  in  size  any  European  species,  attaining  even 
fifty  feet  in  lengtho 


Plesiosaurs, 

13  J 

species. 

Dinosaurs, 

21 

Crocodilians 

,14 

Pterosaurs, 

7 

Chelonians, 

48 

Mosasaurs. 

50 

Total, 

153 

(< 

Fig.  319.— Hypsilophodon,  x  ^.    (Restored  by  Marsh.) 


356 


HISTORICAL   GEOLOG  Y. 


The  Dinosaurs 
were  also  abun- 
dant, and  of  great 
size.  The  re- 
stored skeleton  of 
a  European  spe- 
cies (Fig.  319)  will 
give  some  idea  of 
their  general  ap- 
pearance and  size. 

The  Ptero- 
saurs, or  winged 
saurians,  of 
America,  were  of 
enormous  size  in 
the  Cretaceous, 
some  attaining 
an  alar  extent  of 
twenty-five  feet ; 
but  a  striking  pe- 
culiarity of  them 
was  the  entire  ab- 
sence of  teeth. 
On  this  account 
they  have  been 
put  into  a  distinct 
family,  Pterano- 
donta  (toothless, 
winged).  The 
Pteranodon  in- 
gens  had  tooth- 
less jaws,  four 
feet  long,  and 
wings  twenty-two 
feet  from  tip  to 
tip  (Fig.  320). 


MESOZOIC  ERA.— AGE  OF  REPTILES. 


357 


But  the  Mosasaui's  were  the  most 
abundant  and  also  the  most  character- 
istic of  all,  being  found  only  in  the  Cre- 
taceous. At  least  fifty  species  are  known, 
and  the  remains  of  1,400  are  now  in  the 
Peabody  Museum  at  Yale  University. 
These  were  long,  slender,  almost  snake- 
like in  form,  with  limbs  in  the  form  of 
powerful  paddles  (Fig.  321).  They  were, 
therefore,  entirely  marine  in  habits,  and 
wholly  incapable  of  locomotion  on  land. 
The  head  was  slender,  and  armed  with 
large,  recurved  teeth.  They  were  allied 
most  nearly  to  lizards,  and  therefore 
might  be  called  huge  sea-lizards ;  but, 
like  most  early  animals,  they  were  a  gen- 
eralized type,  connecting  also  with  other 
orders,  especially  snakes.  Some  species 
were  seventy  to  eighty  feet  long,  and 
had  teeth  seven  inches  in  length. 

Birds. — The  history  of  the  discovery 
of  fossil  birds  is  interesting.  In  1862 
the  wonderful  Jurassic  bird,  ArcTiCBop- 
teryx,  already  spoken  of  (page  338),  was 
discovered.  But  this  stood  alone,  with- 
out links  connecting  it  with  typical 
birds.  In  1870  commenced  the  wonder- 
ful series  of  discoveries  by  Marsh,  mostly 
in  the  Cretaceous  of  the  West,  which 
served  largely  to  fill  up  this  gap. 
About  twenty  species  of  Cretaceous  birds 
have  been  described  by  him.  Of  these, 
about  one  half  were  ordinary  water- 
hirds,  allied  to  the  Rails,  Divers,  Cor- 
morants, etc.,  though  of  different  gen- 
era,  but  the  other  ten  were  wonderful 


358 


HISTORICAL   GEOLOGY. 


Toothed-iirds,  wholly  different  from  anything  now  living. 
These  Toothed-birds  were,  again,  of  two  types.  Those 
of  the  one  class  (of  which  the  Uesjperornis  may  be  taken 


Pig.  322.— Ichthyornis  victor, 
(Restored  by  Marsh.) 


Fig.  323.— Hesperornis  regalis,  x  ^. 
stored  by  Marsh.) 


(Re- 


as  a  type)  were  flightless  swimmers  and  divers,  of  great 
size  (five  to  six  feet  long),  with  scarcely  a  rudiment  of 
wings.  Those  of  the  other  class  (of  which  the  Ichthy- 
ornis is  the  type)  were  smaller  in  size,  but  powerful  fliers. 
The  HesperornidcB  had  teeth  in  grooves — a  lower  condi- 
tion. The  IchthyornidcB  had  teeth  Lot  in  distinct  sockets. 
We  give  herewith  (Figs.  322,  323)  Marshes  restorations  of 
these  two  types. 

Mammals. — We  found  marsupials  somewhat  abundant 
in  the  Jurassic,  though  no  true  typical  mammals.  It  is, 
therefore,  somewhat  remarkable  that  no  mammal  of  any 
kind  has  yet  been  found  in  the  Cretaceous,  except  in  the 
Laramie,  which  may  be  regarded  as  a  transition  to  the 


^ 


MESOZOIG  ERA.— AGE  OF  REPTILES.         .359 

Tertiary.  Yet,  doubtless,  marsupials  did  exist  throughout 
the  Cretaceous,  because  they  existed  in  the  Jurassic,  and 
again  in  the  Tertiary,  and  even  now ;  and  it  is  a  law  in 
paleontology  that  a  form,  once  become  extinct,  is  never 
revived.  Nature  never  repeats  herself.  Doubtless,  mar- 
supials existed  in  some  part  of  the  earth,  and  their 
remains  will  yet  be  discovered. 

General  Ohservations  on  the  Mesozoic, 

That  this  was,  in  a  most  wonderful  degree,  an  age  of 
reptiles,  is  easily  shown.  In  the  world,  at  the  present 
time,  there  are  about  six  great  reptiles — one  crocodile  in 
Africa,  two  gavials  in  India,  three  alligators  in  America, 
North  and  South — all  of  them  in  tropical  and  sub-tropical 
regions,  and  none  more  than  twenty  to  twenty-five  feet 
long.  Now,  take  a  single  epoch,  the  AVealden — compar- 
able, therefore,  with  the  present — and  only  the  small 
area  of  England.  There  were  in  England,  in  Wealden 
times,  five  or  six  dinosaurs,  twenty  to  sixty  feet  long ; 
ten  or  twelve  marine  saurians  and  crocodilians,  ten  to 
fifty  feet  long,  besides  pterodactyls,  turtles,  etc.  Again, 
in  America,  in  Cretaceous  times,  leaving  out  the  turtles, 
there  were  more  than  one  hundred  species  of  land,  marine, 
and  flying  reptiles,  the  larger  number  of  which  were 
greater  than  any  living  crocodile.  In  the  epoch  of  the 
Atlantosaur  beds,  reptiles  were  probably  as  numerous, 
and  certainly  of  still  greater  size.  These  are  the  known ; 
but,  of  course,  the  findings  are  but  a  small  fraction  of 
the  actual  fauna.  The  fact  is,  reptiles  were  rulers  in 
every  realm  of  Nature.  They  stood  in  place  of  beasts, 
as  rulers  of  the  land  ;  of  whales  and  sharks,  as  rulers  of 
the  sea  ;  and  in  place  of  birds,  as  rulers  of  the  air.  They 
impressed  their  reptilian  character — the  fashion  of  the 
court — on  all  other  higher  classes ;  the  mammals  were 
reptilian,  and  so  were  the  birds. 


360  HISTORICAL  GEOLOGY, 

Disturbances  whicli  closed  the  Cretaceous  Period 
and  Mesozoic  Era. — Kemember  that  during  the  Creta- 
ceous a  great  sea.,  stretching  from  the  Gulf  of  Mexico  to 
the  Arctic  Ocean,  covered  the  whole  Plains  and  Plateau 
region,  and  divided  the  continent  into  two  continents — 
an  eastern  and  a  western.  Now,  at  the  end  of  the  Creta- 
ceous, this  great  sea  was  abolished  by  the  gradual  upheaval 
of  this  region,  and  the  continent  became  one.  At  the 
same  time  the  western  marginal  bottom  of  the  great 
interior  sea  yielded  to  horizontal  pressure,  and  was 
crushed  together  and  swelled  up  into  the  Wahsatch 
Range.  At  the  same  time,  also,  the  Colorado  Moun- 
tains, which  had  been  a  line  of  islands  in  the  Cretaceous 
sea  (map,  page  349),  were  pushed  up,  and  the  Cretaceous 
strata  sharply  uptilted  on  the  flanks.  At  the  same  time, 
also,  the  Uintah  Mountains  seem  to  have  been  born. 
Such  great  changes  in  physical  geography  imply  corres- 
ponding changes  in  climate,  and  in  fauna  and  flora.  We 
ought  to,  and  do,  indeed,  find  the  animals  and  plants 
very  different  in  the  next  age  (Cenozoic). 

Laramie  or  Transition  Epoch, 

The  abolition  of  the  great  Cretaceous  sea,  and  the 
unification  of  the  continent,  as  we  have  said,  were  pro- 
duced by  the  upheaval  of  the  Plains  and  Plateau  region. 
When  completed,  the  Plateau  region  was  occupied  by 
great  fresh-water  lalces,  which  we  shall  describe  hereafter. 
But  this  change  took  place  gradually,  passing  througli 
intermediate  stages  of  brackish-water  seas.  When  marine 
conditions  prevailed,  it  was  undoubtedly  Cretaceous ; 
when  fresh-water  conditions  were  established,  it  was 
undoubtedly  Tertiary.  But  what  shall  we  call  the  inter- 
mediate time  of  brackish  water  ?  This  is  evidently  a 
transition  period.  It  is  the  lost  interval  between  the 
Cretaceous  and  the  Tertiary  in  Europe,  recovered  here. 


MESOZOIC  ERA.— AGE  OF  REPTILES.  361 

As  we  might  expect,  we  find  Cretaceous  types  lingering 
and  Tertiary  types  coming  in,  and  the  two  coexisting 
side  by  side.  The  Cretaceous  dinosaurs  linger,  but  the 
Tertiary  plants  are  introduced.  The  paleo-zoologists  are 
disposed  to  ally  it  with  the  Cretaceous,  and  the  paleo- 
botanists  with  the  Tertiary.  It  is  really  a  transition  be- 
tween the  two. 

Plants. — Vegetation  was  luxuriant  at  this  time.  More 
than  three  hundred  species  of  dicotyls  have  been  described 
here.  But,  as  the  types  are  wholly  Tertiary,  we  shall  illus- 
trate them  under  that  head. 

Coal. — The  conditions  seem  to  have  been  favorable, 
not  only  for  luxuriant  vegetation,  but  for  its  preserva- 
tion as  coal,  and  nearly  all  the  Cretaceous  coal  mentioned 
on  page  351  belong  to  this  transition  period,  and  have 
therefore  been  often  put  in  the  Tertiary.  Next  to  the 
Coal-measures,  this  is  the  great  coal-bearing  period  of 
the  United  States.  The  largest  fields  are  in  the  Plains 
and  Plateau  region,  viz. :  1.  A  large  field,  the  Marshall 
coal-field,  in  western  Kansas,  about  5,000  square  miles. 

2.  Another  large  field,  in  New  Mexico,  of  equal   size. 

3.  A  third  large  field,  in  Dakota,  extending  into  British 
America.  4.  A  large  and  valuable  field,  in  the  Plateau 
region,  on  the  Laramie  Plains,  stretching  through  Wyo- 
ming to  the  borders  of  Utah.  These  altogether  can  not 
be  less  than  50,000  square  miles.  On  the  Pacific  slope 
several  coal-fields,  probably  of  the  same  age,  are  found  : 
1.  Mount  Diablo  and  Corral  Hollow  field.  2.  Seattle, 
Carbon  Hill,  and  Bellingham  Bay  field.  3.  Nanaimo  or 
Wellington  field  on  Vancouver's  Island.  Coal  is  also 
found  in  Arizona  and  in  southern  California,  but  the  age 
is  not  known. 

All  the  later  coals  are  often  called  lignites,  but  much 
of  it  is  an  excellent  coal,  scarcely  distinguishable  from 
carboniferous  coal.  We  herewith  jDresent  in  tabulated 
form  all  the  principal  coal-fields  of  the  United  States  * 


362 


HISTORICAL   GEOLOGY. 


Carboniferous. 


Jura-Triassic. 


Laramie. 


(  Appalachian. 

J  Central. 

"j  Western. 

'  Michigan. 

(  Eastern  Virginia. 

(  North  Carolina. 

j  Plains  and  Plateau. 

\  Pacific  Slope. 


192,000. 

[  500  (?). 
[  50,000  (?). 


Animals — Reptiles. — A  large  number  of  the  most 
extraordinary  reptiles  yet  discovered  (Fig.  324),  and  also 
several  species  of  small  marsupial  mammals,  are  found  in 
these  uppermost  Cretaceous  beds. 


Fig.  324.— Triceratops  prorsus,  x  ^.    (After  Mareh.) 


CHAPTER  V. 

OENOZOIC   ERA. — AGE   OF  MAMMALS. 

This  is  reckoned  a  primary  division — an  Era — ^because 
there  is  just  here  a  very  general  break  in  the  rock-system, 
and  a  very  great  change  in  the  life-systein.  It  is  also 
called  an  Age,  because  a  new  and  higher  dominant  class 
appears  here.  In  Europe,  the  unconformity  is  universal, 
and,  as  might  naturally  be  expected,  there  is  an  appa- 
rently sudden  change  in  the  life-system.  But  in  America 
the  Laramie  is  not  only  everywhere  conformable  with  the 
Cretaceous  beneath,  but  in  many  places  also  with  the 
Tertiary  above  ;  so  that  the  record  is  almost  continuous. 
And  yet,  at  the  same  level,  viz.,  between  the  Laramie 
and  the  Tertiary,  we  find  an  enormous  change  of  life- 
forms.  It  is  impossible  to  account  for  this,  unless  we 
admit  that  the  steps  of  progress  were  quicker  at  this 
time. 

General  Characteristics. — In  a  geological  sense, 
modern  history  commences  here.  Modern  types  of  ani- 
mals and  plants,  modern  aspects  of  field  and  forest,  were 
fairly  inaugurated.  Now  was  established  in  broad  outline 
the  present  order  of  things — the  present  rulers  on  lan'S 
(except  man),  in  the  seas,  and  in  the  air  ;  the  present 
adjustment  of  the  orders  of  animals  and  plants.  Hence 
the  name,  ^^  Cenozoic.'^  Some  of  these  characteristics, 
however,  especially  the  introduction  of  Dicotyls,  and 
therefore  the  aspect  of  forests,  were  anticipated  in  the 
Cretaceous.      ^  s  there  is  now  a  new  and  higher  dominant 

363 


364 


HISTORICAL   GEOLOGY. 


class,  viz.,  mammals,   reptiles  must   decline  in  number 
and  size,  and  thus  seek  safety  in  a  subordinate  position. 

Subdivisions. — The  Cenozoic  era  and  Mammalian  age 
is  divided  into  two  periods — Tertiary  and  Quaternary, 
In  the  Tertiary  all  the  mammalian  species  are  extinct, 
but  many  invertebrate  species  are  still  living,  and  the 
percentage  of  living  species  increases  with  time.  In  the 
Quaternary,  on  the  contrary,  nearly  all  the  invertebrate 
species,  e.  g.,  mollusks,  still  survive,  and  some  of  the 
mammalian  species  also  survive.  These  facts  are  shown 
in  the  diagram  (Fig.  325).  The  space  above  the  lines 
of  mollusks  and  mammals  shows  proportion  of  extinct. 


Fig.  325.— Diagram  showing  the  relative  number  of  species  living  and  extinct. 

and  below  the  line,  of  living,  species.  The  dawn  of  liv- 
ing species  of  shells  is  with  the  beginning  of  the  Ter- 
tiary ;  the  dawn  of  living  mammalian  species  is  in  the 
Quaternary.  Both  curves  show  increasing  percentage 
of  living  species  with  time. 


Section  I. — Tertiary  Period. 

As  already  stated,  the  dawn  of  living  molluscan  species 
is  in  the  earliest  Tertiary,  and  thenceforward  the  per- 
centage of  living  species  steadily  increases ;  but  no  living 
mammalian  species  are  found  there. 

Subdivisions. — The  subdivisions  of  the  Tertiary  period 
into  epochs  are  founded  on  this  percentage  of  l.'ving  mol- 
luscan species.  It  is  thus  divided  into  three  epochs — 
Eocene,  Miocene,  and  Pliocene.     If  we  find  a  stratum 


C:EN0Z0IC  era.— age  of  mammals.  365 

which  contains  not  more  than  5  to  10  or  15  per  cent,  of  its 
shells  still  living  in  neighboring  seas  or  lakes,  we  call  it 
Eocene  ;  if  15  to  40  or  50  per  cent.,  we  call  it  Miocene  ;  if 
50  to  80  or  90  per  cent.,  we  call  it  Pliocene.  This  is 
graphically  illustrated  in  the  diagram  (Fig.  325). 

r  Pliocene,  50  to  90  per  cent,  living. 
Tertiary  Period.  \  Miocene,  15  "  50         '*  " 

(  Eocene,      5  "  15        "  " 

Roch' System, 

Areas  in  the  United  States. — 1.  On  the  Atlantic 
border,  going  south,  we  find  no  Tertiary  until  we  reach 
New  Jersey.  Thence  to  Georgia  there  is  a  band  of  Ter- 
tiary strata  about  a  hundred  miles  wide,  resting  in  'New 
Jersey  on  the  Cretaceous,  but  elsewhere  against  the 
Archaean  gneiss.  It  constitutes  what  are  called  the  low 
countries  of  the  Southern  Atlantic  States.  The  rivers, 
in  passing  from  the  gneissic  to  the  softer  Tertiary,  make 
falls  or  rapids.  Here,  therefore,  is  the  head  of  naviga- 
tion of  the  Southern  rivers,  and,  therefore,  also  the  posi- 
tion of  many  important  towns.  Richmond  and  Peters- 
burg, Virginia ;  Raleigh,  North  Carolina ;  Columbia, 
South  Carolina  ;  Augusta,  Milledgeville,  and  Macon, 
Georgia — are  thus  situated. 

2.  The  same  broad  strip  of  Tertiary  lowlands  borders 
the  Gulf,  resting  there,  however,  on  the  Cretaceous  (see 
map,  page  272),  expands  northward  to  the  mouth  of  the 
Ohio  River,  and  sweeps  southward  about  the  western  bor- 
der of  the  Gulf  into  Mexico. 

3.  On  the  Pacific  border  we  find  Tertiary  with  Creta- 
ceous, forming  the  Coast  ranges  of  California  and  Ore- 
gon. All  these  border  Tertiaries — Atlantic,  Gulf,  and 
Pacific — are  marine  deposits. 

4.  But  in  the  interior  regions — i.  e..  Plains,  Plateau, 
and  Basin — we  have  extensive  fresh-water  deposits.    Some 


366  HISTORICAL  GEOLOGY. 

of  these  are  Eocene,  some  Miocene,  some  Pliocene.  The 
Eocene  deposits  are  in  the  Plateau  region  north  and  south 
of  the  Uintah  Mountains.  The  Miocene  and  Pliocene 
deposits  are  in  the  Plains  and  the  Basin  regions. 

These  fresh-water  deposits  of  the  West  are  imperfectly 
lithified,  and  therefore  are  sculptured  by  erosion  into  the 
curious  forms  called  Mauvaises  Tcrres,  as  already  ex- 
plained (page  248,  Fig.  152). 

Physical  Geography. — It  is  easy,  from  the  distribu- 
tion just  given,  to  reconstruct  the  physical  geography  of 
the  American  Continent  during  the  Tertiary.  It  is  sim- 
ply a  restatement  in  another  form  of  what  we  have  already 
said.  On  the  Atlantic  border  the  New  England  shore-line 
was  farther  out  than  now,  because  we  have  no  Tertiary, 
deposits  exposed  along  that  coast.  The  Tertiary  shore- 
line crossed  the  present  shore-line  about  New  York,  and 
thence  passed  along  the  line  of  limit  of  the  Tertiaries  of 
the  Southern  Atlantic  States,  the  waves  beating  there 
against  Archaean  shore-rocks.  On  the  Gulf  border  the 
north  shore  of  the  Gulf  did  not  reach  quite  so  far  as  in 
Cretaceous  times  (see  map  on  page  272),  but  the  Gulf 
waters  covered  all  the  flat  lands  about  the  Gulf,  beating 
here  on  Cretaceous  rocks ;  extending  north,  as  an  embay- 
ment  to  the  mouth  of  the  Ohio  River,  and  then  swept 
southward,  covering  a  broad  strip  on  the  west.  The  Up- 
per Mississippi  (if  it  existed  at  all)  and  the  Ohio  emptied 
by  separate  mouths  into  the  embayment.  On  the  Pacific 
border  the  waves  of  the  Pacific  beat  against  the  foot-hills 
of  the  Sierra,  the  place  of  the  Coast  Eange  being  then  a 
marginal  sea-bottom.  Fig.  326  shows  the  American  con- 
tinent in  Early  Tertiary. 

The  interior  region  was  occupied  by  enormous  lakes. 
During  the  Eocene,  the  lakes  were  in  the  Plateau  region; 
during  Miocene  and  Pliocene  times,  in  the  Basin  on  the 
one  side  and  the  Plains  on  the  other. 

Coal. — Lignite  is  found  again   in  the  Tertiary,  espe- 


CENOZOIC  ERA.—AOE  OF  MAMMALS. 


3G7 


cially  in  the  Miocene.    The  Coos  Bay  coal  of  Oregon,  and 
the  imperfect  seams  of  the  Contra  Costa  Range,  Califor- 


nia, are  Miocene.  The  lone  brown  coal  of  Amador 
County,  California,  is  still  more  recent,  probably  Plio- 
cene. 


368  BISTOIUCAL  OEOLOOY. 

Life-Sydem, 

General  Character. — This  era  is  called  Cenozoic  be- 
cause modern  life  iii  its  main  features  commences  here. 
We  are  tlierefore  prepared  to  find  that,  among  i)lants  and 
lower  animals,  the  general  similarity  to  present  forms  is 
so  great  that  the  difference  would  hardly  be  recognized 
by  the  popular  eye.  We  must  touch  very  lightly  on  these 
lower  forms. 

Plants. — We  have  already  seen  that  in  the  Cretaceous 
many  familiar  genera  of  forest-trees  were  introduced.  In 
fact,  so  far  as  trees  are  concerned,  the  Cenozoic  might  be 
said  to  commence  in  the  Cretaceous.  In  the  Tertiary 
nearly  all  the  genera  are  the  same  as  now,  although  the 
species  are  mostly  different.  The  genera  are  the  same  as 
now,  hut  not  in  the  same  localities.  On  the  contrary,  the 
same  genera  grew  much  farther  north  than  now.  The 
vegetation  indicated  a  much  warmer  temperature  tlian 
now.  In  Eocene  times,  palms  and  other  tropical  plants 
grew  all  over  Europe,  and  the  mean  temperature  seems  to 
have  been  75°  to  80°.  In  Miocene  times,  evergreens,  like 
those  now  about  the  shores  of  the  Mediterranean,  flour- 
ished even  to  Lapland  and  Spitzbergen.  The  mean  tem- 
perature of  Europe  was  10°  to  20°  higher  than  now. 

In  America,  during  the  Eocene,  paints  and  figs  and 
evergreens,  in  Dakota,  show  a  temperature  there  about 
that  of  Florida  now.  In  Miocene  times.  Sequoias  very 
like  the  Big  tree  and  the  Redwood  of  California,  and 
taxodiums,  and  magnolias — almost,  if  not  quite,  identical 
with  the  cypress  of  the  Southern  swamps  and  the  Mag- 
nolia gYandiflora  of  Southern  forests — were  abundant  in 
Greenland.  The  temperature  of  Greenland  was  then  at 
least  30°  higher  than  now.  It  is  easy  to  see  that  polar 
ice  could  not  have  existed,  and  Arctic  expeditions  would 
have  been  an  easy  matter,  if  man  had  lived  at  that  time. 
We  give  some  figures  of  Tertiary  plants  (Figs.  327-332). 


CENOZOIC  ERA.— AGE  OF  MA3I3IALS. 


369 


But  if  these  highest  plants  were  exceptionally  abun- 
dant, so  were  also  the  lowest  of  all,  vi:.c^  the  unicelled 


Fig.  asr. 


Fig.  320. 


Pig.  330. 


Fig.  331. 


^  Fig.  3]2. 

Figs.  327- 3:^2. —Ainericaii  Tertiary  plants  (after  Safiford  aii<l  Losquereux) :  827.  Quer- 
ciis  crassinervis.  328.  Andromeda  vaccinifoliie  afflnis.  329.  Carpolithes  irregu- 
laris. 330.  Fagus  ferruginea— nut.  331.  Fruit  of  Sequoia  Langsdorfii  (after 
Heer).    332.  Leaf  of  Sequoia  Langsdorfii  (after  Heer). 


diatoms.  The  great  deposits  of  diatomaceous  earths 
found  in  many  parts  of  the  Avorld  are  Tertiary.  In  the 
United  States  the  best  known  localities  are  near  Rich- 
mond, Virginia,  and  in  California.  These  deposits  are 
many  miles  in  extent,  and  thirty  to  one  hundred  feet 
thick,  and  made  up  wholly  of  the  silicious  shells  of  these 
microscopic  plants. 

Le  Conte,  Geol.  24 


370  HISTORICAL   GEOLOGY, 

Animals, 

The  similarity  in  general  appearance  of  most  Tertiary 
invertebrates  to  living  species  is  so  great  that  we  shall 
only  draw  brief  attention  to  a  few  interesting  points. 

Mollusca. — We  are  all  doubtless  interested  in  the  fam- 
ily/ history  of  the  oyster.  The  family  commenced  in  the 
Jurassic,  increased  in  the  Cretaceous,  and  culminated  in 
the  Tertiary,  and  then  declined.  The  Ostrea  Georgiensis 
and  the  CaroUnensis  of  the  Eocene  were  several  times 
larger  than  their  modern  representative.  The  Ostrea 
Titan,  of  the  Pacific  coast  Miocene,  was  still  larger,  being 
thirteen  inches  long,  eight  inches  wide,  and  six  inches 
thick.  Lest  some  may  regret  inconsolably  the  passing 
away  of  these  magnificent  oysters  before  the  advent  of 
man,  I  hasten  to  remind  them  that  what  has  been  lost  in 
size  has  probably  been  gained  in  flavor. 

Insects. — Insects  are  always  closely  associated  with 
land  vegetation,  and  the  kinds  of  the  one  are  determined 
by  the  nature  of  the  other.  Now,  for  the  first  time,  the 
highest  flowering  plants  are  abundant,  and  now,  for  the 
first  time  also,  all  orders  of  insects,  even  the  highest 
flower-loving  kinds,  such  as  butterflies,  bees,  ants,  etc., 
are  abundant  (Fig.  333).      On   account   of  the   greater 


Fig.  333.— Ants  and  bees  of  European  Miocene.    (After  Heer.) 

warmth  and  moisture,  both  vegetal  and  insect  life  were 
fuller  even  than  now.  We  select  a  few  examples  of  find- 
ings, by  means  of  which  we  may  reproduce  in  imagina- 


CENOZOIC  ERA.— AGE  OF  MAMMALS.  371 

tion  the  conditions  of  things  which  prevailed  in  Tertiary 
times : 

1.  In  the  Miocene  fresh-water  deposit  of  Oeningen,  a 
layer  two  feet  thick  is  black  with  the  remains  of  insects. 
It  is  also  full  of  leaves.  About  nine  hundred  species  of 
insects  and  five  hundred  species  of  plants  have  been  made 
out.  The  larger  number  of  insects  are  beetles  and  ants. 
We  may  imagine  that  in  Miocene  times  there  was  at 
Oeningen  a  lake  surrounded  with  a  thick  forest,  whose 
leaves  were  scattered  on  the  waters  and  cast  upon  the 
shore.  Beetles  and  flying  ants,  essaying  to  fly  over  the 
lake,  were  beaten  down  by  the  winds  and  also  cast  on 
the  shore.  These  remains  were  covered  up  by  mud,  and 
thus  preserved. 

2.  On  the  shores  of  the  Baltic,  bits  of  amber,  derived 
from  Miocene  strata  outcropping  beneath  the  water,  are 
continually  thrown  up  by  the  action  of  the  waves.  In 
these  are  found,  sealed  up,  and  in  transparent  pieces 
clearly  visible,  great  numbers  of  insects,  often  in  an 
exquisitely  perfect  state  of  preservation.  About  eight 
hundred  species  of  insects  and  one  hundred  and  fifty 
species  of  plants  have  been  described.  The  insects  are 
mostly  winged  ants  and  flies.  Amber  is  known  to  be  the 
fossil  gum  of  a  pine  {Pinus  succinifer).  We  may  imagine, 
then,  that  in  Miocene  times,  in  the  region  now  occupied 
by  the  southern  Baltic,  there  was  a  forest,  among  the 
trees  of  which  the  Pinus  succinifer  abounded.  From 
these  trees  a  semi-liquid,  sticky  gum  exuded  in  tears,  on' 
which  insects  alighting  stuck  fast,  and  were  covered  by 
later  exudations. 

3.  In  Auvergne,  France,  there  is  a  Miocene  fresh-water 
deposit,  one  layer  of  which,  two  to  three  feet  thick,  is 
almost  wholly  composed  of  the  cast-off  cases  {indusia)  of 
caddis-worms,  and  is  therefore  called  indusial  limestone. 
The  caddis-worm  (larva  of  the  caddis-fly)  of  to-day  is  a 
wingless  creature,  living  wholly  in  the  water.     It  has  the 


&n 


HISTORICAL   GEOLOGY. 


curious  habit  of  gathering  bits  of  wood,  small  dead  shells, 
or  even  grains  of  sand,  and  webbing  them  together  to 
form  a  cylindrical  hollow  case  in  which  it  lives.    When  it 


Fig.  334.— Fragment  of  indusial  lime- 
stone (natural  size),  showing  the 
caddis-worm  cases. 


Fig.  335— Recent  caddis-worm, 
with  its  case. 


wishes  to  walk  about,  it  puts  out  the  head  and  legs  for 
that  purpose,  as  seen  in  the  figure.  These  cases  are  left 
when  the  worm  changes  into  the  caddis-fly.  We  may 
imagine,  then,  that  in  Auvergne,  in  Miocene  times,  there 


Pig.  ;i36.— Prodryas  Persephone.    (After  Scudder.) 


was  a  lake  in  which  lived  countless  generations  of  caddis- 
worms,  and  their  cast-off  cases  accumulated  until  a 
deposit,  two  to  three  feet  thick,  was  produced. 


CENOZQIC  EIIA.—AQE   OF  MAMMALS.  373 

4.  Only  very  recently  a  remarkable  American  locality 
has  been  discovered.  At  Florissant,  Colorado,  a  fresh- 
water deposit  of  Upper  Eocene  or  Lower  Miocene  age  has 
been  found,  one  layer  of  which  is  black  with  the  remains 
of  insects  of  all  kinds.  Scudder  has  identified  more  than 
a  thousand  species.  We  give  here  (Fig.  336)  a  beauti- 
fully preserved  butterfly  from  this  locality.  Here,  then, 
we  have  phenomena  like  those  at  Oeningen,  and  explained 
in  the  same  way. 

Fishes. — In  general  appearance,  Tertiary  fishes  are 
much  like  those  of  the  present  day.  Then,  as  now,  Tele- 
osts  vastly  predominated  (Fig.  337),  and  Ganoids  were 


Pig.  ;i37.  -Tertiary  fishes— Teleosts  :  Lebias  cephalotes,  Miocene. 

nearly  extinct.  Then,  as  now,  sharks  were  among  the 
chief  rulers  of  the  seas.  In  fact,  they  seem  to  have  cul- 
minated in  the  Tertiary.  The  Eocene  strata  of  the  At- 
lantic border  are  in  places  full  of  sharks'  teeth,  some  of 
which  are  of  incredible  size.  We  have  seen  one  of  these, 
of  the  kind  represented  in  Fig.  338,  which  would  more 
than  cover  a  page  of  this  book,  being  nearly  seven  inches 
long  and  six  inches  wide.     The  original  possessors  of  such 


374 


HISTORICAL   GEOLOGY. 


teeth  could  hardly  liave  been  less  than  sixty  to  seventy 
feet  long. 

Reptiles. — The  reign  of  reptiles  is  past.    The  Eeptilian 

dynasty  is  overthrown. 
This  class  no  longer  oc- 
cupies a  prominent  place 
in  history.  In  geological 
history  the  ruling  class 
is  always  the  fittest  to 
ule,  which  can  not  al- 
ways be  said  of  the 
reigning  families  in  hu- 
man history. 

The  great  char- 
acteristic reptiles 
of  the  Mesozoic 
are  all  extinct. 
Among  great  rep- 
tiles, the  crocodili- 
ans  alone  remain. 
The  reptiles  of 
the  Tertiary  are 
of  the  same  fam- 
ilies as  now  exist, 
viz.,  crocodiles,  turtles,  snakes,  lizards,  frogs,  toads,  and 
salamanders.  Snakes  seem  a  low  type,  and  yet  were  intro- 
duced only  in  the  Tertiary.  But  they  are  not  low  in  the 
sense  of  undevelojyed.  They  have  developed  backward — 
they  are  an  example  of  a  degraded  type.  The  tailless  am- 
phibians (frogs  and  toads)  are  undoubtedly  the  highest 
among  amphibians  ;  for  they  pass  through  the  tailed  stage 
(tadpole)  in  embryonic  life.  These  tailless  amphibians 
were  introduced  first  in  the  Tertiary.  The  biggest  known 
turtle  ( Colossochelys)  was  found  in  the  Miocene  of  India. 
Its  shell  was  twelve  feet  long,  eight  feet  wide,  and  seven 
feet  high. 


-Tertiary    fishes— Sharks :    Carcharodon 
megalodon,  x  ^.    (After  Gibbes.) 


CENOZOIC  ERA.- AGE   OF  MAMMALS. 


375 


Birds. — It  will  be  remembered  that  the  earliest  bird 
known  (the  Jurassic  Archgeopteryx)  was  also  the  most 
reptilian.  In  the  Cretaceous 
we  found  both  reptilian 
toothed-hirds  and  ordinary 
water-birds.  Now,  in  the 
Tertiary,  as  in  the  present, 
all  the  reptilian  birds  had 
disappeared,  and  only  typi- 
cal birds  remain  ;  and  not 
only  water-birds,  but  also  the 
highest,  viz.,  land-birds.  In 
other  words,  the  bird-class 
had  now  fairly  separated  it- 
self from  the  reptilian,  and 
the  connecting  links  were 
all  destroyed. 

Nearly  all  the  families  of 
birds  now  existing  have  been 
found  in  the  Tertiary,  but 
also  a  few  of  strange  forms. 
The  Gastornis,  of  the  Eocene 
of  Paris  (Fig.  339),  was  a 
huge  bird,  ten  feet  high,-and 
a  curious  connecting  link  be- 
tween waders  and  ostriches.  Besides  these  curious  forms, 
many  birds  have  been  found,  in  the  Tertiary  of  this  coun- 
try and  in  Europe,  similar  to  those  still  living.  But  in 
France,  especially,  the  birds,  like  the  plants  and  insects, 
show  a  decided  tropic  climate.  Parrots,  trogons,  ibises, 
secretary-birds,  and  flamingoes  inhabited  France  at  that 
time. 

Mammals. 

Remember  that,  although  marsupials  or  reptilian  mam- 
mals were  found  in  Jura-Trias,  and  doubtless  continued 


Fig.   339.— Restoration    of     Gastornis 
Eduardsii.    (After  Meunier.) 


376  HISTORICAL  OEOLOOY, 

through  the  Cretaceous,  true,  ordinary,  or  typical  mam- 
mals first  appear  in  the  lowest  Tertiary,  and  immediately 
became  the  dominant  class. 

Some  Preliminary  Remarks. — Before  describing 
the  Tertiary  mammals,  there  are  some  points  requiring 
notice  : 

1.  The  suddenness  of  their  appearance  is  very  remark- 
able. In  the  very  lowest  Tertiary,  without  warning  and 
without  apparent  progenitors,  true  mammals  appear  in 
great  numbers,  in  considerable  diversity,  and  even  of  the 
highest  order — Primates,  or  monkey  tribe.  Now,  in  Eu- 
rope, where  there  is  a  decided  break  and  a  lost  interval, 
this  is  not  so  surprising  ;  but  even  in  America,  where  the 
Laramie  passes  without  break  into  the  Tertiary,  the  same 
is  true.  At  a  certain  level  the  great  dinosaurs  disappear, 
and  the  mammals  take  their  place.  A  new  dynasty  and  a 
new  age  in  history  commence.  It  is  impossible  to  account 
for  this  by  natural  causes,  unless  we  admit  times  of  rapid 
progress.  In  addition  to  this,  we  must  also  admit  that 
the  apparent  sudden  appearance  in  a  particular  place  is 
largely  due  to  migration. 

2.  We  have  said  that  they  appeared  in  great  numbers 
and  considerable  diversity.  All  the  great  branches  of  the 
Mammalian  class  were  represent'ed  in  the  first  fauna — 
herbivores,  carnivores,  and  primates  or  monkeys.  Yet 
these  were  not  so  distinctly  separated  as  now.  TTiey  were 
all  generalized  types.  If  we  represent  all  the  orders  and 
families  of  mammals  as  branches  and  sub-branches  of  one 
main  trunk,  then,  as  we  go  backward  in  time,  these  be- 
come less  numerous  and  less  widely  separated.  In  the 
earliest  Eocene  the  branches  are  few  and  very  near  to- 
gether. The  carnivores  are  but  slightly  separated  from 
the  herbivores — in  fact,  they  are  both  omnivores.  The 
monkeys,  also,  were  not  yet  fairly  sex)arated  as  typical 
monkeys.  They  are  therefore  called  Prosimiae,  or  pro- 
genitors of  the  true  monkey.    Manifestly,  i^  these  branches 


CENOZOIC  ERA— AGE  OF  MAMMALS.  377 

liiive  a  common  origin,  it  must  be  sought  still  lower,  prob- 
ably in  the  Laramie, 

Tertiary  Lake-Deposits  of  the  West, 

Nowhere  in  the  world  is  there  so  complete  a  series  of 
Tertiary  deposits  and  of  Tertiary  mammals  as  in  the  lake- 
deposits  of  the  Plateau  and  Plains  and  Basin  regions  al- 
ready spoken  of  (page  365).  We  shall  therefore  take  most 
of  our  illustrations  from  these. 

Eocene  Lake-I>eposits. — In  the  Lower  Eocene  de- 
posits— viz.,  Puerco  beds  and  Wahsatch  or  Coryphodon 
i)ed3 — have  been  found  nearly  one  hundred  species  of 
mammals,  including  carnivores,  herbivores,  insectivores, 
and  monkeys.  Perhaps  the  most  remarkable  and  charac- 
teristic animals  of  the  lower  Tertiary  were  the  Corypho- 
donts.  These  were  huge  animals  with  very  small  brains, 
plantigrade  feet,  slow,  awkward  movements,  and  very 
generalized  structure. 

In  the  Middle  Eocene  Bridger  beds,  mammalian  life 
was  even  still  more  abundant.  More  than  one  hundred 
species  are  known,  and  these  are,  of  course,  but  a  fraction 
of  what  actually  existed.  Perhaps  the  most  remarkable 
animals  of  this  time  were  those  of  the  Dinoceras  family. 
The  Dinoceras  may  be  taken  as  a  type  of  the  family. 
This  was  a  heavy-built,  sluggish-moving  animal  of  ele- 
phantine size,  with  a  most  singular  conformation  of  head, 
which  was  armed  with  three  pairs  of  horns  and  a  pair  of 
huge  tusks,  as  shown  in  Fig.  340.  Some  are  supposed 
to  have  had  a  head  five  feet  long. 

During  the  Eocene,  also.  Marsh  finds  the  earliest  pro- 
genitors of  the  horse.  In  the  early  Eocene  is  found  the 
Eohippus,  an  animal  which  liad  three  hoofed  toes  on  the 
liind-feet  and  four  perfect  hoofed  toes  and  a  rudimen- 
tary fifth  toe  on  the  fore-feet.  This  was  followed  in  the 
Middle  Eocene  by  the  Orohippus,  similar  to  the  other. 


378  HISTORICAL   GEOLOGY. 

except  that  the  fifth  rudimentary  toe  is  dropped.     These 
animals  were  about  the  size  of  a  fox. 


Fig.  340.— Tinoceras  ingens.    (After  Marsh.) 

Miocene. — The  Eocene  lake-deposits  are  in  the  Pla- 
teau region,  the  Miocene  and  Pliocene  are  in  the  Plains 
and  Basin  region.  The  first  thing  to  be  noted  here  is  the 
complete  change  of  species.  It  is  worthy  of  note  that  in 
the  Miocene  many  existing /amt7/es  (not  species),  such  as 
the  rhinoceros  family,  the  camel  family,  the  deer  family, 
the  dog  family,  and  the  cat  family  commenced  to  exist. 
A.mong  the  many  forms  which  occur  here  we  can  only 
mention  the  most  remarkable.  In  this  respect,  certainly, 
the  Brontothere  stands  first.  This  animal  was  still  larger 
than  the  tinoceras,  and  connects  the  latter  with  the 
rhinoceros.  The  peculiar  saddle-shaped  head  was  three 
feet  long.  It  had  only  three  toes  behind,  like  the  rhi- 
noceros, but  four  in  front. 

In  the  Miocene  the  horse  family  is  represented  by  the 
Mesohippus  and  Miohippus.  These  had  three  toes  on 
the  hind  and  the  fore  foot,  and  were  about  the  size  of  a 
sheep.     True  tridactyl  horses  commence  here. 


CENOZOIG  URA.—AGE  OF  MAMMALS.  379 

Pliocene.— Here,  again,  we  have  a  great  change  of 
mammalian  species.  The  animals  are  much  more  like 
existing  species.     If  many  existing  families  commenced 


.  341.— Brontops.    (After  Marsh.) 


in  the  Miocene,  we  find  many  existing  genera,  such  as  the 
horse  {equus),  the  camel  (camelus),  the  elephant  (elephas), 
etc.,  commencing  in  the  Pliocene.  Great  numbers  of  the 
horse  family,  Protohippus,  FUoMppus,  and  finally  Equus, 
great  numbers  of  the  camel  family,  several  elephants  and 
mastodons,  roamed  in  herds  over  the  American  Continent. 
The  Protohippus  was  a  three-toed  horse,  like  the  Miohip- 
pus,  but  the  side- toes  were  shorter.  It  was  very  similar 
to  the  Hipparion  of  Europe  (Fig.  343).  The  Pliohippus 
was  very  horselike.  It  was  one-toed,  like  a  true  horse, 
the  two  side4oes  having  dwindled  to  splints. 

Foreign  Localities. 

Paris  Basin. — Among  foreign  Tertiary  deposits  the 
most  celebrated  is  the  Eocene  basin  of  Paris.  The  streets 
of  Paris  teem  with  a  living  generation  of  men  and  animals. 


380  IITSTOlilCAL   GEOLOGY. 

Its  cemeteries  are  full  of  the  remains  of  a  former  genera- 
tion. But  a  little  deeper  down  we  find  another  cemetery 
full  of  the  remains  of  extinct  aiiiinals  of  strange  forms. 
The  masterly  study  of  this  fauna  by  the  illustrious  Cuvier 
gave  an  incredible  impulse  to  geology.  One  striking  char- 
acteristic of  this  fauna  was  the  great  predominance  of 
tapirlike  animals.  Of  fifty  species  of  mammals  found 
here,  forty  species  were  of  this  general  kind.  The  most 
celebrated  of  these  remains  are  the  Paleothere  (Fig.  342) 


Fig.  342.— Paleotherium  magnum,  X  ^^.    (After  Gaudry.) 

and  the  Anoplothere.  The  Paleothere  was  a  three-hoofed 
animal  allied  to  the  tapir,  and  perhaps  connecting  with 
the  horse  family.  The  Anoplothere,  on  the  contrary,  was 
a  two-hoofed  animal,  apparently  connecting  tapirs  with 
the  ruminants.  In  these  two  we  have  the  even-toed  and 
the  odd-toed  hoofed  animals  almost  united.  The  great 
bird  Gastornis,  figured  on  page  375,  was  found  here. 

It  is  probable  that  during  the  Eocene  the  Paris  basin 
was  the  place  of  an  estuary,  and  the  bodies  of  animals 
of  that  epoch  were  washed  down  by  a  river  and  buried 
in  sediments  at  its  mouth. 

In  the  European  Miocene  groat  numbers  of  remains 


CE NO  ZOIC  ERA.— AGE   OF  MAMMALS, 


381 


have  been  found.  Corresponding  with  the  Miohippus  and 
perhaps  the  Protohippns  of  the  United  States,  was  the 
graceful  tridactyl  horse  (Hipparion),  represented  in  Fig. 
343.     The  most  remarkable  animal  of  this  time  was  the 


Fig.  343.— Skeleton  of  Hipparion  gracile,  restored.    (After  Gaudry.) 


huge  Dinothere,  the  earliest  of  the  Proboscidians.  It  had 
a  proboscis,  but  not  yet  developed  to  the  size  and  strength 
which  this  organ  attained  in  the  mastodon  and  the  ele- 
phant. The  singular  form  of  the  head  is  shown  in  Fig. 
344.  True  monkeys  were  introduced  in  the  Miocene,  and 
that  most  destructive  of  carnivores,  the  saber-toothed 
tiger  {Machairodits),  in  the  Pliocene,  though  the  genus 
culminated  in  the  Quaternary  (see  Fig.  356,  page  402). 

Some  General  Observations  on  the  Tertiary  Mam- 
mals ;  Genesis  of  Mammalian  Orders  and  Families, 
etc. — We  have  already  said  that  in  the  earliest  Eocene, 
the  great  branches  of  the  mammalian  class  were  very 
near  together,   though  their  point  of  union  has  not  yet 


382  HISTORICAL  GEOLOGY. 

been  found.  As  time  went  on,  these  separated  more  and 
more  widely,  and  gave  off  sub-branches,  which  again 
divided,  and  so  on.  In  general  terms,  it  may  be  said  that 
some  of  the  existing  orders  may  be  traced  back  to  the 

Eocene.  Many  of  the  exist- 
ing families  commenced  in 
the  Miocene  ;  existing  genera 
in  the  Pliocene ;  but  existing 
species  only  in  the  Quater- 
nary. This  is  well  illustrated 
by  one  great  branch,  the  Un- 
gulates, or  hoofed  animals. 
These  consist  now  of  many 
widely  separated  sub- 
branches  ;  but  in  the  earliest 

Fig.  344. -Head    of    Dinotheriam       T^j.^-^^.       ^j^^       ^^^^    ^^    ^^^^-^^ 
giganteum,  greatly  reduced.  .  '^  -^    . 

into  one,  a  primal  ungulate. 
As  we  go  up,  this  branch  separates,  even  in  the  Upper 
Eocene,  into  odd-toed  (perissodactyls)  and  even-toed 
(artiodactyls)  ungulates.  In  the  Miocene,  each  of  these 
again  separates,  the  former  into  the  elephant  family 
(Proboscidians)  with  live  toes,  the  tapir  and  rhinoceros 
families  with  three  toes,  and  the  horse  family,  with  three 
toes  passing  into  07ie  ;  the  latter  into  the  hog  and  hippo- 
potamus families  with  four  toes,  and  the  ruminant  family 
(horned  animal^)  with  two  toes. 

Genesis  of  the  Horse. — Let  us  trace  one  of  these 
branches  throughout.  We  select  for  this  purpose  the 
horse.  A  most  wonderful  series  representing  this  family, 
about  forty  species  in  all,  has  been  furnished  by  the 
American  Tertiaries,  and  the  successive  steps  traced  by 
Professor  Marsh.  First  of  all,  in  the  early  Eocene  Wah- 
satch  beds,  we  find  the  Eohippus  (dawn-horse).  This 
little  animal  (the  size  of  a  fox)  had  three  toes  on  the 
hind-foot,  and  four  perfect  toes  and  a  fifth  splint,  and 
perhaps  dew-claw,  on  the  fore-foot.    Next,  in  the  Middle 


CENOZOIC  ERA.— AGE  OF  MAMMALS. 

b  c  d  e  f 


383 


Bquus  Qua- 
ternary and 
Recent. 


Pliohippus : 
Pliocene. 


Protohippus  • 
Lower  Pllo- 


Miohippus : 
Miocene. 


Mfcsohippus  • 
Lower  Mio- 
cene. 


Orohippus . 
Eocene. 


Pig.  345.— Diagram  illustrating  gradual  changes  in  the  horse  family.  Throughout, 
a  is  fore- foot ;  6,  hind-foot ;  c,  fore-arm  ;  c?,  shank  ;  e,  molar  on  side-view ; 
/  and  <y,  grinding  surface  of  upper  and  lower  molars.    (After  Marsh.) 


384  HISTORICAL   OEOLOQY. 

Eocene,  came  the  Orchippus,  about  the  same  size,  with 
three  toes  behind  and  four  in  front — the  fifth  splint  being 
dropped.  Next,  in  the  Miocene,  came  the  Mesohippus 
and  the  Miohippus  (about  the  size  of  a  sheep),  with  three 
toes  behind  and  in  front,  but  the  fourth  toe  of  the  Oro- 
hippus  still  retained  as  a  useless  splint.  In  these  the 
horse  family  may  be  said  to  be  fairly  established.  Then, 
in  the  Lower  Pliocene,  came  the  Protohippus,  about  the 
size  of  an  ass,  with  three  toes  on  all  the  feet,  but  the  two 
side-toes  shorter,  and  the  mid-toe  larger,  than  before. 
Then,  lastly,  in  the  uppermost  Pliocene,  come  the  Plio- 
hippus  and  Equus,  in  which  the  side-toes  are  reduced  to 
useless  splints,  and  the  middle  toe  is  greatly  enlarged. 
This  is  the  case  in  the  modern  horse  ;  its  side-splints  attest 
its  three-toed  ancestry. 

Crust- Movements  during  and  closing  the  Tertiary  Period, 

Remember  that,  during  the  Cretaceous,  a  great  sea 
covered  the  whole  of  the  Plains  and  Plateau  region, 
dividing  the  continent  into  two  continents.  By  the 
gradual  elevation  of  the  region,  this  sea  was  obliterated 
and  replaced  by  great  lakes.  The  formation  of  these 
lakes  inaugurated  the  Tertiary.  The  elevation  of  the 
same  region  continuing,  these  Tertiary  lakes  were  suc- 
cessively obliterated,  and  the  prodigious  general  erosion 
and  canon-cutting  of  this  region  commenced. 

On  the  Pacific  border,  at  the  end  of  the  Miocene,  the 
Coast  Range  of  California  and  Oregon  was  born.  From 
the  beginning  of  the  Cretaceous,  the  place  of  this  range 
had  been  marginal  sea-bottom  receiving  sediment.  At 
the  end  of  the  Miocene,  these  yielded  to  horizontal  pres- 
sure, were  crushed  together,  and  swelled  up  into  this 
great  range.  Probably  at  the  same  time  occurred  the 
great  lava-flood  of  the  northwest,  described  on  page  218. 

Ou  the  Atlantic  border  the  changes  were  far  less 
i-emarkable.     There  was,  however,  a  gradual  increase  of 


1 


CENOZOIC  ERA.—AQE  OF  MAMMALS.  386 

the  land  along  the  border,  until,  at  the  end  of  the  Ter- 
tiary, the  continent  was  finished,  except  the  southern 
part  of  Florida  and  its  keys,  and  a  very  narrow  strip 
along  the  Southern  coast  generally.  The  southern  point 
and  the  keys  of  Florida  are  still  growing  (see  page  111). 

Section"  IL — Quaternary  Period. 

This  is  one  of  the  most  interesting  and  yet  most  diffi- 
cult portions  of  the  history  of  the  earth.  It  is  the  last 
period  preceding  and  preparatory  to  the  present. 

Characteristics. — The  grand  characteristic  of  this  pe- 
riod is  the  occurrence  of  wide-spread  up-and-down  move- 
ments of  the  earth's  crust  in  high  latitudes  or  circum- 
polar  regions  north  and  south,  attended  with  great 
changes  of  climate  from  extreme  rigor  to  temperateness, 
and  consequent  great  changes  in  species.  Also,  the  age 
of  mammals  seems  to  culminate  here,  and  man  appears 
on  the  scene,  and  was  doubtless  an  important  agent 
among  others  in  bringing  about  the  change  of  species. 
Nearly  all  the  invertebrate  species  and  some  mammals  of 
the  Quaternary  are  still  living.  A  small  percentage  of 
the  present  mammalian  species,  man  among  the  number, 
commenced  here  (see  Fig.  325,  page  364). 

Subdivisions. — The  Quaternary  period  is  divided  into 
two  epochs,  founded  upon  the  attitude  of  the  land  and 
the  changes  of  climate.  These  are — 1.  Glacial.  2.  Cham- 
plain.  The  Glacial  epoch  was  characterized  by  upward. 
crust-movement  in  high-latitude  regions,  until  the  land 
there  stood  2,000  to  3,000  feet  higher  than  now,  was 
sheeted  with  ice,  and  an  Arctic  rigor  of  climate  extended 
in  America  almost  to  the  shores  of  the  Gulf.  The  Cham- 
plain  epoch  was  characterized  by  a  downward  movement 
in  the  same  region  until  the  land  was  500  to  1,000  feet 
lower  than  now,  so  that  many  lower  parts  of  the  conti* 
nent  were  covered  with  sea  ;  and  by  a  moderation  of  tern'- 

Le  Conte,  Geol.  25 


386  HISTORICAL  GEOLOGY. 

perature,  a  melting  of  ice,  and  a  flooding  of  lakes  and 
rivers.  It  was  therefore  a  flooded  epoch.  Loosened 
icebergs  floated  over  the  flooded  seas  and  lakes.  It  was 
therefore,  also,  an  epoch  of  the  reign  of  icebergs.  From 
this  condition  the  crust  gradually  rose  again  to  the  pres- 
ent condition  of  things. 

Similar  changes  seem  to  have  occurred  everywhere  in 
high-latitude  regions,  but  we  are  not  sure  that  they  were 
absolutely  contemporaneous.  Therefore  it  will  be  best  to 
take  the  whole  series  of  changes  right  through  for  each 
locality.  We  commence  with  the  Eastern  United  States, 
because  it  has  been  best  studied  there. 

Quaternary  ik  Easterit  North  America. 

1.  Glacial  Epoch, 

The  Drift. — The  phenomena  now  about  to  be  de- 
scribed are  extremely  varied ;  but,  as  they  exist  all  over 
the  Northern  United  States,  we  insist  that  every  one 
observe  for  himself.     What  we  say  is  meant  only  as  a 


All  over  the  northern  portion  of  our  country,  from  38° 
to  40°  latitude  northward,  mantling  over  hill  and  dale, 
over  mountain  and  valley,  is  found  a  peculiar  deposit  or 
soil  composed  of  a  heterogeneous  mixture  of  earth,  gravel, 
pebbles,  and  rock-fragments  of  all  sizes.  As  this  material 
has  evidently  been  shifted  and  sometimes  brought  from  a 
long  distance,  it  is  called  Drift.  It  is  impossible  to  make 
a  description  which  will  apply  to  all  cases,  but  almost 
everywhere  the  lower  part  in  contact  with  the  bed-rock 
consists  of  stiff  clay  with  disseminated  stones  rounded  or 
partly  rounded,  and  scratched  (Fig.  346).  This  is  called 
the  stony-clay  or  bowlder-clay.  It  is  exactly  like  the 
ground  moraine  of  a  glacier,  mentioned  on  page  58.  In 
places  are  found  heaps  or  dumps  of  loose  materials  sim- 
ilar to  the  top  moraine  of  glaciers.     In  places  the  ma- 


CENOZOIC  ERA.^AGE  OF  MAMMALS.  387 

terials  may  be  irregularly  stratified  and  cross-laminated, 
as  if  by  water  running  beneath,  or  from  the  snout  of  a 
glacier.     In  places  the  laminae  may  be  twisted  and  crum- 


Pio.  846.— Subangular  Btone.    (After  Gelkie.) 

pled,  as  if  by  a  glacier  pushing  along  on  a  mud  surface. 
In  still  other  places,  especially  west  of  the  Appalachian, 
the  upper  part  is  more  widely  stratified.  But  this  may 
belong  to  a  later  epoch  (Champlain). 

Bowlders. — Over  all  are  scattered  rock-fragments  and 
bowlders,  of  all  sizes,  both  angular  and  rounded — some- 
times as  thick  as  hailstones  after  a  storm,  and  actually 
cumbering  the  earth.  These  bowlders,  whether  imbedded 
in  drift  or  scattered  on  the  surface,  are  usually  entirely 
different  from  the  country-rock.  Great  blocks,  of  thou- 
sands of  cubic  feet,  are  often  seen  perched  where  they  do 
not  belong,  as  if  stranded  by  glacier  or  iceberg.  The  par- 
ent ledge  from  which  they  were  torn  can  often  be  found, 
and  thus  the  direction  of  their  transport  is  known.  By 
this  means  it  has  been  ascertained  that  from  the  Cana- 


388  HISTORICAL   GEOLOGY. 

dian  highlands  the  material  has  been  carried  southeast- 
ward, southward,  and  southwestward.  The  distance  car- 
ried has  been  in  some  cases  several  hundred  miles. 

Bed-Rock  Surface. — Wherever  the  drift-mantle  is 
removed,  the  bed-rock  underlying  is  found  to  be  glaciated, 
i.  e.,  it  presents  a  smooth,  billowy  surface,  scored  with 
straight  parallel  marks,  precisely  like  the  pathway  of  a 
glacier,  described  on  page  Gl.  The  general  direction  of 
these  marks  is  the  same  as  that  of  the  transport  of  the 
bowlders,  viz.,  southeast,  south,  and  southwest. 

Southern  Limit  of  the  Drift ;  Ice-Sheet  Moraine. 
— The  most  characteristic  of  the  phenomena  described, 
viz.,  the  stony  clay,  the  glaciated  bed-rock,  and  the  great 
bowlders,  extend  over  the  whole  northern  portion  of  the 
continent,  down  to  about  38°  to  40°  north  latitude. 
Along  this  southern  limit  are  found  remnants  of  the 
terminal  moraine  of  the  ice-sheet. 

Its  position  is  marked  on  the  map  by  the  strong  line. 
Within  this,  and  marked  on  the  map  by  the  dotted  lines, 
another  and  later  and  far  distincter  terminal  moraine  is 
seen  sweeping  about  the  Great  Lakes  and  westward  in 
huge  festoons  (Fig.  347). 

Explanation. — The  simplest  explanation  of  these  facts 
is,  that  during  this  epoch  the  whole  northern  part  of  the 
continent  was  elevated,  so  that  the  Canadian  highlands 
were  1,000  to  2,000  feet  above  its  present  level,  and  com- 
pletely covered  with  an  ice-mantle  several  thousand  feet 
thick,  as  Greenland  and  the  Antarctic  Continent  are  to- 
day. This  ice-mantle,  covering  everything  except  per- 
haps the  highest  peaks,  moved  southeastward,  southward, 
and  southwestward,  scoring  the  whole  surface  of  the  coun- 
try in  its  path,  and  accumulating  bowlders  and  earth  be- 
neath it.  At  its  limit,  represented  by  the  strong  line  seen 
on  the  map,  the  accumulations,  being  more  abundant, 
formed  a  moraine.  After  a  while,  the  ice-limit,  by  melt- 
ing, went  northward,  dropping  bowlders  in  its  course  to 


CENOZOIC  ERA.— AGE  OF  MAMMALS. 


389" 


or  perhaps  beyond  the  lakes,  but  again  advanced,  and 
formed  the  deeply  lobed  moraine  marked  by  the  dotted 
lines. 

AYe  have  given  only  the  limit  of  the  general  ice-sheet. 
But  in  mountain-regions,  e.  g.,  in  Colorado,  and  perhaps 


Fig.  347.— Map  showing  limit  of  the  drift  and  the  second  ice-sheet  moraine.  Limit 
of  northern  drift  represented  by  heavy  line  from  Long  Island  to  Minnesota ; 
second  ice-sheet  moraine  represented  by  triple  dotted  line. 


in  Virginia,  even  beyond  this  limit,  there  were  great 
separate  glaciers,  occupying  the  valleys,  as  shown  by  the 
moraines  left  by  them.  The  tracing  of  the  course  of 
these  old  glaciers  by  their  glaciated  pathways,  perched 


390  HISTORICAL   GEOLOGY, 

bowlders,  and  terminal  and  lateral  moraines  gives  a  fas- 
cinating interest  to  travel  among  these  mountains. 

Contrast  of  Northern  and  Southern  Soils  and 
Rock-Surfaces. — Nothing  can  be  more  striking  than  the 
contrast  between  the  soil  and  underlying  rock-surfaces 
within  and  beyond  the  limits  of  the  Drift.  Within  these 
limits  the  covering  is  a  heterogeneous  mass  of  shifted  ma- 
terial lying  on  sound  rock  ;  south  of  this  limit  the  soil  is 
stratified,  and  in  many  places  graduates  into  the  rock  be- 
neath from  which  it  has  been  formed  by  rotting  in  place. 
Again,  the  underlying  rock  in  drift-regions  is  glaciated, 
i.  e.,  smooth,  moutonneed,  scored;  beyond  the  drift-jregion 
there  is  either  no  distinct  surface  to  the  rock,  or  elsCj  if 
there  be,  it  is  a  rough,  weathered  surface. 

2.   Ghamplain  Epoch. 

At  the  end  of  the  Glacial  epoch,  when  the  condition 
of  things  was  such  as  described  above,  there  commenced 
a  crust-movement  in  a  contrary  direction,  by  which  the 
land  in  the  same  region  was  brought  downward  100  to 
500  or  1,000  feet  below  this  present  level,  and  the  lower 
parts  of  the  continent  became  covered  with  the  sea.  It 
was  therefore  a  period  of  inland  seas.  The  movement  was 
attended  with  moderation  of  temperature,  by  which  the 
ice-sheet  was  melted  and  progressively  retired  northward. 
The  melting  ice  produced  flooded  lakes  and  flooded  rivers. 
It  was  therefore  also  2^,  flooded  period.  Icebergs,  loosened 
from  the  northern  ice-foot,  floated  over  the  inland  seas 
and  the  great  flooded  lakes,  dropping  debris.  Some  of 
the  great  bowlders  are  probably  to  be  accounted  for  in 
this  way.  It  was  therefore  also  a  period  of  iceberg  agency. 
The  evidences  of  this  condition  of  things  are  found  in  old 
elevated  sea-margins,  lake-margins,  and  old  river  flood- 
plain  deposits. 

Sea-Margins. — Elevated  sea-beaches  are  found  in  all 
countries  affected  with  the  Drift.     The  highest  one  marks 


CENOZOIC  ERA.— AGE  OF  MAMMALS. 


391 


the  level  in  the  Champlain  period.  In  southern  New 
England  it  is  50  feet  high,  in  Maine  100  feet  high,  on 
the  Gulf  of  St.  Lawrence  and  Labrador  500  feet,  and 
in  Greenland  1,000  feet  high.  The  old  sea-line  may 
be  traced  on  both  sides  of  the  St.  Lawrence  River,  and 
thence  around  Lake  Champlain  nearly  500  feet  high, 
showing  that  there  was  a  wide  bay  or  sound  in  this  re- 
gion. It  is  this  which  gives  name  to  the  epoch.  On 
the  bench  marking  the  sea-level  about  Lake  Champlain 
have  been  found  not  only  many  marine  shells,  but  also 
the  skeleton  of  a  stranded  whale. 

Lake-Margins. — About  all  the  Great  Lakes  are  found 
now  many  terraces  or  benches  rising  one  above  another, 
the  highest  marking  the  greatest  extent  of  the  lake. 
About  Ontario  the  highest  is  500  feet ;  about  Lake  Erie, 
250  feet ;  about  Lake  Superior,  330  feet.  These  lakes 
doubtless  at  that  time  ran  together,  forming  a  vast  sheet 
of  water  which  drained  southward  through  the  Mississippi 
River  into  the  Gulf.  At  the  same  time  an  enormous  lake 
covered  the  region  about  Lake  Winnipeg  and  drained 
through  the  Minnesota  River  into  the  Mississippi.  This 
ancient  lake  has  been  called  Lake  Agassiz. 

River-Deposits. — The  section.  Fig.  348,  represents  in 


'•1  ■■ 

-\       5 

i,     r    /^= 

^_y<r^ 

1" 

if-r-f^       , 

^  R    '1  1 

\\ 

^_.]^>^ 

01    " 

»    )      ap   '     ,  ..     ">.^ 

—--^r 

a "       1 

n 

1 

— — y'    „  1 

,,.  1 . 

'/      1      /I            1.            (1             '   t 

„  1 . .'.  I 

Fig.  348.— Ideal  section  across  river-bed  iu  drift-rcKion. 


a  general  way  the  condition  of  the  rivers  in  all  the  drift- 
region.    Beneath  the  present  river-bed,  r,  there  is  a  much 


392  HISTORICAL  GEOLOGY. 

wider  and  deeper  old  river-bed,  R  R,  which  is  filled  up 
often  several  hundred  feet  deep  with  river-silt,  h  h,  and 
into  this  the  river  is  now  cutting  its  bed.  The  great  river- 
bed, R  R,  was  cut  out  during  the  epoch  of  elevation  (Gla- 
cial) and  previous  periods.  They  are  preglacial  river- 
beds. Th.Q  filling  was  done  during  this  epoch  of  subsi- 
dence (Champlain).  The  river  since  then  has  again  cut 
down,  but  not  so  deeply.  All  the  rivers  in  the  drift- 
region,  therefore,  are  bordered  on  each  side  by  a  wide 
area  of  old  silt,  usually  much  above  the  present  flood- 
level,  and  therefore  forming  high  bluffs  or  terraces, 
sometimes  one,  sometimes  many,  on  each  side. 

The  Cause  of  the  flooded  condition  was  primarily  the 
great  water-supply  from  melting  of  the  ice-sheet.  But  it  is 
evident  that  the  subsidence  of  the  land  would  cause  the 
sea  to  enter  the  mouths  of  many  rivers,  forming  great 
estuaries  ;  and  also,  by  diminishing  the  slope  of  the  river- 
bed, would  tend  to  increase  their  floods. 

From  this  subsided  condition  the  land  gradually  rose 
again,  by  successive  stages,  to  the  present  condition. 
These  successive  stages  are  marked  by  a  succession  of 
sea-beaches,  lake-terraces,  and  river-terraces,  below  the 
highest  just  described.  As  the  land  rose,  successive  sea- 
margins  were  left ;  the  outlet  of  the  lakes  also  cut  deeper 
and  deeper,  and  drained  the  lakes  to  lower  and  lower 
levels.  Also,  all  the  rivers  cut  deeper  and  deeper  into 
the  old  Champlain  silts,  leaving  them  as  bluffs  and  ter- 
races high  above  the  present  flood-line  (Fig.  348).  Some- 
times there  is  but  one  great  bluff  on  each  side,  as  in  the 
Mississippi  Kiver.  Sometimes  there  are  several  terraces, 
one  above  the  other,  as  in  the  case  of  the  Connecticut 
River.  It  is  evident  that  when  Lake  Champlain  was  first 
cut  off  from  the  sea  by  elevation  it  was  a  salt  lake.  It 
was  freshened  in  the  manner  explained  on  page  79. 


CENOZOIC  ERA.^AQE  OF  3IAMMALS.  393 


Quaternary  in  the  Western  Part  of  the  Continent. 

On  the  Pacific  slope  the  signs  of  all  these  movements 
are  clear  ;  especially  are  the  signs  of  extensive  glaciation 
magnificent.  "We  shall  again  vary  our  mode  of  presenta- 
tion by  tracing  the  condition  of  things  throughout  the 
Quaternary  in  seas,  glaciers,  lahes,  and  rivers.  We  take 
seas  first,  because  by  this  we  establish  the  oscillations. 

Seas. — A  more  elevated  condition  of  land  than  now 
exists  is  plainly  shown,  not  only  by  the  boldness  of  the 
Western  coast  and  the  existence  of  a  line  of  bold,  rocky 
islands  a  little  way  off  shore,  a  recognized  sign  of  a  sunken 
coast,  but  also  by  the  remarkable  fact  that  remains  of  the 
Quaternary  mammoth  have  been  found  on  one  of  these 
islands — the  Santa  Rosa.  When  this  elephant  lived,  the 
island  was  evidently  connected  with  the  mainland. 

A  subsequent  subsided  condition  is  demonstrated  by 
sea-margins  in  many  places.  We  shall  describe  briefly 
the  condition  of  the  sea.  At  that  time  the  Bay  of  San 
Francisco  was  enormously  enlarged ;  for  its  waters  covered 
the  whole  of  the  flat  lands  about  the  bay,  including  the 
Santa  Clara,  Napa,  and  Sonoma  Valleys,  and  then,  passing 
through  the  Straits  of  Carquinas,  spread  all  over  the  great 
interior  valley  of  California  (Sacramento  and  San  Joaquin), 
forming  an  inland  sea  fifty  miles  wide  and  three  hundred 
miles  long.  The  old  beach-marks  may  be  traced  in  many 
places.  Lake  Tulare  is  a  remnaut  of  this  great  inland 
sea.  In  Oregon  the  sea  went  up  the  Columbia  Eiver, 
and  spread  over  the  Willamette  Valley,  forming  a  great 
sound.  From  this  subsided  condition  the  land  rose 
again,  making  successive  terraces  down  to  the  present 
level. 

Glaciers. — It  is  still  doubtful  if  the  general  ice-sheet 
extended  on  this  coast  as  far  south  as  California,  although 
abundant  evidences  are  found  in  British  Columbia ;  but  it 


394 


HISTORICAL   GEOLOGY. 


is  certain  that  the  whole  Sierra  was  at  that  time  covered 
with  perpetual  snow,  from  which  ran  great  glaciers  forty 
to  fifty  miles  long  to  the  valleys  below.  It  is  certain  that 
all  the  valleys  and  canons  which  trench  the  flanks  of  the 
Sierra  were  filled  with  glaciers  of  enormous  size.     Many 


Fig.  349.— Glaciated  surface  and  scattered  bowlders  near  Lake  Tenaya,  Cal. 
(From  a  photograph  by  J.  N.  Le  Conte.) 


of  these  have  been  traced  in  the  clearest  manner  by  their 
polished  pathways,  their  scattered  bowlders,  and  their 
lateral  and  terminal  moraines  (Fig.  349). 

Lakes. — All  the  lakes  of  that  time,  especially  in  the 
Basin  region,  were  greatly  enlarged.  About  Lake  Mono, 
terraces  rise,  one  above  another,  to  700  feet  above  the 
present  lake-level,  and  inclosing  an  immense  area.  The 
lake-waters  then  washed  against  the  foot  of  the  Sierra, 
and  glaciers  ran  into  its  waters  and  produced  icebergs. 
At  the  same  time,  the  whole  lower  part  of  the  Utah  and 
Nevada  basins  was  filled  each  with  a  great  lake.  That 
which  filled  the  Utah  basin,  called  Lake  Bonneville,  was 


ClENOZOIC  ERA.— AGE  OF  MAMMALS. 


395 


100  miles  wide  and  300  miles  long.  The  traveler  on  the 
Union  Pacific  Railway  can  hardly  fail  to  observe  the  old 
terraces,  rising  up  to  1,000  feet  above  the  present  lake- 
level.  It  drained  at  that  time  into  the  Snake  and  Co- 
lumbia Rivers,  then  lost  its  outlet,  and  dried  away  to  the 
remnants — Great  Salt  Lake,  Utah  Lake,  and  Sevier  Lake 
— which  we  now  have.  The  lake  which  filled  the  Nevada 
basin — Lalce  Lahontan — was  of  nearly  equal  size,  and  its  . 
dried-away  residues  are  seen  in  numerous  salt  and  alkaline  \ 
lakes,  such  as  Pyramid,  Winnemucca,  Humboldt,  Carson, 
Walker,  etc.,  which  overdot  this  great  area. 

Rivers. — The  old  or  preglacial  river-beds,  on  the 
eastern  side  of  the  continent,  as  we  have  seen  (page  391), 
underlie  the  present  river-beds — i.  e.,  are  in  the  same 
place,  but  deeper.  In  middle  California  the  relation  is 
quite  different  and  peculiar.  Here  the  old  river-beds 
overloolc  the  new — i.  e.,  they  are  in  a  different  place,  and 
higher..  The  old  river-beds  are  on  the  divides  between 
the  new.  The  reason  is  this  :  In  middle  California,  at 
the  beginning  of  the  Glacial  epoch,  the  old  river-beds  had 
already  been  filled  up,  first  with  gravel,  and  then,  by 
igneous  outbursts,  with  lava.  The  rivers  were  thus  dis- 
placed, and  began  to  cut  new  beds.     But  at  the  same 


Pig.  350. — Ideal  section  through  two  modem  river-beds  and  table-mountain  divide ; 
r't  old  river-bed ;  r,  r,  present  river-beds ;  »,  elate ;  gr,  new  gravel ;  ;,  lava ; 
gr'^  old  gravel  under  the  lava. 


time  there  was  a  considerable  lifting  of  the  whole  moun- 
tain-region, and  consequently  the  rivers  now  cut  deeper 
than  before  (Fig.  350).  Thus  it  has  come  to  pass  that 
the  new  river-beds  occupy  the  places  of  the  old  divides. 


396  HISTORICAL  GEOLOGY, 

and  the  old  river-beds  are  now  found  on  the  top  of  the 
present  divides. 

Phenomena  similar  to  those  discussed  are  found  in 
Europe  and  in  all  other  high-latitude  regions,  both  north 
and  south  of  the  equator. 

8ome  General  Results  of  Glacial  Erosionc 

lo  Fiords. — If  one  examines  an  accurate  map  of  coast- 
lines^  he  will  see  that,  in  the  region  affected  by  Quater- 
nary oscillations,  there  is  a  bold,  deeply  dissected  coast- 
linoc  In  Norway  these  deep  inlets  are  called  fiords^ 
and  therefore  this  structure,  wherever  found,  is  called 
fiord-structure.  We  find  it  strongly  marked  in  Green- 
land and  in  Alaska.  This  structure,  in  Norway,  is 
partly  due  to  the  action  of  waves  (page  45),  but  also,  and 
mainly,  to  the  submergence  of  old  glacial  valleys.  In 
Greenland  and  Alaska  they  are  still  partly  occupied  by 
glaciers. 

2.  Lakes. — Examine  your  map  of  North  Americao 
See  how  the  whole  northern  part  is  dotted  over  with  lakes, 
while  the  southern  part  is  almost  destitute  of  them.  See 
also  that  the  lake-area  is  also  the  area  of  the  drift.  Now, 
although  lakes  may  be  formed  in  many  ways,  and  exist  in 
all  parts  of  the  world,  yet  undoubtedly  the  small  lakes  at 
least,  which  are  so  thickly  sprinkled  over  the  drift-region> 
have  been  produced  by  glacial  agency. 

There  are  several  ways  in  which  glacial  lakes  were 
formed  :  1.  They  are  sometimes  rock-lasins,  scooped  out  by 
glacial  erosion,  2o  They  are  often  formed  by  the  damming 
of  drainage  waters  behind  old  terminal  moraines.  These 
two  kinds  are  thickly  strewed  all  over  high  mountain- 
regions  in  the  pathways  of  old  glaciers.  Standing  on  the 
crest  of  the  Sierra,  fifty  may  sometimes  be  counted  at 
one  view.  3.  In  flat  regions,  as  in  northern  Minnesota 
and  British  America,  they  are  simply  hollows  produced 


b 


CENOZOIC  ERA.— AGE  OF  MAMMALS.  397 

by  inequalities  of  deposit  of  the  Drift  when  the  ice-sheet 
retreatedc 

Life-System,  of  the  Quaternary, 

Plants  and  Invertebrates. — The  plants  and  inver- 
tebrate animals  were  mostly  identical  with  those  still 
living.  We  dismiss  these,  therefore,  with  one  important 
remark.  Quaternary  species  are  indeed  still  living  ;  not, 
however,  in  the  same  place,  but  much  farther  north.  This 
indicated  that  the  climate  was  much  colder  in  the  Qua- 
ternary than  noiu. 

Mammals. — It  is  only  in  mammals  that  we  find  a 
striking  difference  as  compared  with  the  present  time. 
Those  of  the  Quaternary  are  peculiar,  differing  conspicu- 
ously both  from  the  Tertiary  and  the  living  species.  We 
shall  take  our  first  examples  from  Europe,  as  they  have 
been  best  studied  there. 

Quaternary  Mammals  of  Europe. — In  Europe  they 
are  found  sometimes  in  caves,  where  in  great  numbers 
and  of  all  kinds  they  have  become  entombed  ;  sometimes 
on  river-terraces  and  old  sea-leaches,  where  their  floating 
carcasses  have  been  stranded  and  buried  ;  sometimes  in 
peat-hogs,  where,  venturing  in  search  of  food,  they  have 
mired  and  perished  ;  and  sometimes,  as  in  Arctic  regions, 
in  frozen  soils,  where  whole  carcasses  were  sealed  up,  and 
are  now  found  perfectly  preserved. 

The  Mammalian  Age  culminates  here. — As  already 
said,  the  mammalian  age  seems  to  culminate  in  the 
Quaternary  just  before  its  downfall.  For  example,  in 
England  alone,  during  this  time,  there  lived  a  great 
elephant,  the  mammoth  (Elephas  primigenius),  much 
larger  than  any  now  living  ;  two  species  of  the  rhinoceros 
and  one  of  the  hippopotamus  ;  three  species  of  oxen,  two 
of  which  were  of  gigantic  size  ;  a  wild  horse  ;  several 
species  of  deer,  among  which  were  the  reindeer  and  the 
great  Irish  elk,  a  magnificent  animal,  eleven  feet  high  to 


398  HISTORICAL  GEOLOGY. 

the  top  of  its  elevated  antlers  and  ten  feet  between  their 
tipSc  Of  carnivores  there  were  the  great  cav3-bear,  larger 
than  the  grizzly  ;  a  lion  and  a  tiger  as  large  as  the  African 
lion  and  the  Bengal  tiger  ;  a  saber-toothed  tiger  {Machai- 
rodus),  more  formidable  than  either^  with  its  saber-like 
tusks  projecting  six  to  eight  inches  beyond  the  gums  ; 
hyenas  in  great  abundance  ;  besides  many  smaller  species. 
The  remains  of  man  have  also  been  found  associated  with 
these  extinct  animals. 

Mammoth. — This  great  animal  deserves  more  special 
mention.  During  Quaternary  times,  three  great  elephants 
roamed  in  herds  over  Europe.  The  greatest  of  these — in 
fact  the  greatest  of  all  elephants,  and  the  most  nume]  ous 
at  this  time — was  the  mammoth  {Elephas  primigenius). 
The  remains  of  these  are  found  everywhere,  but  the  most 
perfect  in  Siberia.  Here  perfectly  fresh  carcasses  have 
been  exposed  by  the  undermining,  by  the  river,  of  the 
frozen  bluffs  of  the  river-banks.  The  one  represented 
here  (Fig.  351)  is  in  the  Museum  of  St.  Petersburg.  The 
dried  skin  still  remains  on  the  feet  and  portions  of  the 
head.  It  is  known  from  these  carcasses  that  this  elephant 
was  covered  with  a  thick  wool,  and  over  this  long  hair. 
Unlike  living  elephants,  it  was  adapted  to  endure  cold. 
The  same  was  true  of  the  Quaternary  rhinoceros,  the 
carcasses  of  which  have  also  been  found  preserved  in 
the  same  way. 

Quaternary  Mammals  in  America. — Great  mam- 
mals were  equally  abundant  in  America.  There  roamed 
in  herds  all  over  this  country  one  species  of  the  mastodon 
and  two  species  of  the  elephant,  viz.,  the  Elephas  primi- 
geniuSy  or  mammoth,  and  the  Elephas  Americanus,  There 
were  also  three  or  four  species  of  the  horse,  some  of 
gigantic  size  ;  several  species  of  oxen,  one  of  them  ten 
feet  from  tip  to  tip  of  their  widely  spreading  horns ; 
several  species  of  the  elk,  one  of  them  equal  to  the  great 
Irish  elk,  and  a  great  number  of  gigantic   edentates. 


CE NO  ZOIC  ERA.— AGE  OF  MAMMALS. 


399 


.S5 


o  ,a 


.2-3 


■s  a 
I 

5 


ground-sloths,  and  armadillos.  Carnivores  were  not  so 
abundant  as  in  Europe ;  but  there  were  several  species 
of  the  bear,  a  lion,  and  a  saber-toothed  tiger. 

The  Great  Mastodon. — The  most  perfect  specimens 
of  the  mastodon  have  been  found  in  the  peat-bogs,  where, 
venturing  in   search  of  food,  they  have  become  mired. 


400 


HISTORICAL   OEOLOQY. 


Fig.  858. — Mastodon  Americanus.    (After  Owen.) 


Fig.    353.— Tooth  of  :W 
todon  Americanus. 


Pig.  354,— Molar  tooth 
of  a  Mammoth  (Ele- 
phas  primigenius), 
grinding  surface. 


In  Fig.  352  we  give 
one  of  tlie  most 
perfect  of  these. 
Any  one  can  dis- 
tinguish the  re- 
mains of  the  mas- 
todon from  those 
of  the  mammoth, 
if  the  jaw-teeth 
be  preserved. 
The  difference  is 
shown  in  Figs.  353, 
354.  It  is  doubtful 
which  of  these  two 
animals  was  the 
greater;  but  either 


CENOZOIC  ERA.— AGE   OF  MAMMALS.  401 

was  probably  more  than  twice  the  bulk  of  the  greatest  liv- 
ing elephant. 

Quaternary     Mammals    in     South    America. — We 

shall  mention  here  only  the  most  characteristic.  South 
America  now  is  characterized  by  sloths,  armadillos  (eden- 
tates), and  llamas.  In  Quaternary  times  it  was  similarly 
characterized,  but  the  species  were  gigantic.  Great  ground- 
sloths  and  cuirassed  animals  allied  to  the  armadillo,  but 
bigger  than  an  ox,  had  their  homes  in  South  America,  but 
wandered  northward  into  North  America  as  far  as  Cali- 
fornia and  Pennsylvania.  Among  the  ground-sloths,  the 
best  known  are  the  Megatherium  (great  beast)  and  the 
Mylodon.  The  hugest  of  these  was  the  Megatherium 
(Fig.  355).  This  was  as  big  as  a  rhinoceros,  and  had 
thigh-bones  several  times  the  bulk  of  those  of  an  elephant. 
The  massiveness  of  the  hind-legs,  the  hip-bones,  and  the 
tail,  together  with  the  long  arms  and  prodigious  hands, 
seem  to  indicate  that  the  animal  had  the  power  of  stand- 
ing on  its  hind-legs  while  it  reached  up  to  tear  down 
branches  of  trees  and  feed  upon  them. 


Pig.  355.— Megatherium  Cuvieri. 


Among  the  cuirassed  edentates,  the  best  known  is  the 
Glyptodon,  the  shell  of  which  was  at  least  five  feet  long  ; 


Lk  Conte,  Geol. 


403  HISTORICAL  GEOLOGY, 

but  other  genera  have  been  found  much  larger,  one  as 
big  as  a  rhinoceros,  and  another  as  big  as  an  ox.     The 

saber-toothed  tigers  were  also 
abundant  in  South  America  at 
this  time  (Fig.  356). 

Quaternary  Mammals  of 
Australia. — At  the  present 
time  the  mammals  of  Australia 
are  all  marsupials.  So  was  it 
also  in  Quaternary  times  ;  but 
the  species  were,  again,  gigantic. 
The  Diprotodon,  for  example, 
Pio.  356.— Head  of  Machairodus  was  a  kangaroo  as  big  as  a  rhi- 

ril^rr"""  ^  *■  '^"'  '^°«e'-°«-     ^any   other  gigantic 

species  are  also  found. 
We  see,  then,  that  the  present  distribution  of  mamma- 
lian forms  was  already  established  in  the  Quaternary,  but 
everywhere  the  species  were  gigantic. 

Some  Important  General  Questions. 

1.  Cause  of  the  Cold  of  the  Glacial  Epoch. — The 

intense  cold  which  characterized  the  Glacial  epoch  may 
have  been  due  to  terrestrial  or  to  cosmical  causes.  It 
seems  right  that  we  should,  as  far  as  possible,  account 
for  it  by  terrestrial  causes,  and  resort  to  the  other  only 
if  these  fail,  l^ow,  northern  elevation  would  probably 
produce  great  cold  in  the  northern  hemisphere.  This, 
then,  is  certainly  a  probable  cause.  But  the  effect  has 
seemed  so  great  and  widespread  that  many  think  this 
cause  insufficient,  and  have  therefore  looked  abroad  for 
extra-terrestrial  or  for  cosmical  causes:  Among  the  many 
causes  of  this  kind  which  have  been  proposed,  the  only 
one  which  has  attracted  much  attention  is  that  brought 
forward  by  Mr.  Croll,  which  attributes  it  to  slow  changes 
in  the  form  and  position  of  the  earth's  orbit.  * 

*  For  a  discussion  of  this  subject,  see  "  Elements,"  p.  576. 


CENOZOIC  ERA.— AGE  OF  MAMMALS.  403 

2.  Migrations  during  tlie  Glacial  Epoch  and 
their  Effect  on  the  Geographical  Distribution  of 
Organisms. — The  oscillations  of  the  earth^s  crust  during 
glacial  times  produced  great  changes  in  Physical  Geog- 
raphy, elevation  enlarging  and  subsidence  diminishing 
the  area  of  the  continents.  In  this  manner  gateways 
were  opened  permitting  migrations  from  one  conti- 
nent to  another,  as  for  example  between  North  America 
and  Asia  through  Bering  Straits,  and  between  Europe 
and  Africa  through  the  Mediterranean.  Again,  the  great 
changes  of  climate  from  subtropical  mildness  to  extreme 
arctic  rigor,  and  back  again  to  temperateness,  enforced 
migrations  southward  and  northward,  perhaps  several 
times.  These  migrations,  whether  permitted  or  enforced, 
produced  a  mixing  of  different  faunas  and  floras  on  the 
same  ground ;  and  the  severe  competitive  struggles 
among  them,  together  with  the  great  changes  of  climatic 
conditions,  caused  many  changes,  partly  by  extinction 
and  partly  by  modification.  After  these  migrations, 
minglings,  struggles,  and  consequent  modifications,  the 
resulting  faunas  and  floras  were  again  in  many  cases 
reisolated  in  their  new  homes  by  subsidence.  In  these 
isolated  new  homes  they  have  undergone  slow  changes 
by  evolution  until  the  present  time.  Thus  have  come 
about  the  present  geographical  faunas  spoken  of  in  chap- 
ter iii.,  section  4,  of  Part  I.  (P.  118).  Now,  as  the  Glacial 
epoch  is  a  comparatively  recent  geological  event,  it  is 
evident  that  the  migrations  of  that  time  furnish  a  key 
to  the  present  distribution  of  organisms ;  and  conversely, 
the  present  distribution  of  organisms  is  a  key  to  direction 
of  migrations  during  that  time.  We  give  a  few  striking 
examples  illustrating  this  very  interesting  subject,  and 
completing  the  explanations  given  in  Part  I. 

1.  Alpine  Species. — It  is  a  curious  fact  that  alpine 
species  of  plants  and  insects  (i.  e.,  species  which  live  on 
mountains  near  the  snow  line)  are  very  similar  in  all  parts 


404  HISTORICAL  GEOLOGY, 

of  the  world  (as  for  example  in  North  America  and 
Europe),  although  they  are  so  far  separated  from  one 
another  and  completely  isolated.  It  must  be  observed, 
however,  that  they  are  also  very  similar  to  Arctic  species. 
The  explanation  is  found  in  the  migrations  of  the  glacial 
times.  At  that  time  Arctic  species  were  pushed  south- 
ward on  both  continents — to  the  shores  of  the  Mediter- 
ranean in  one  and  of  the  Gulf  of  Mexico  in  the  other. 
On  the  return  of  a  temperate  climate  most  of  them  fol- 
lowed the  retreating  ice-foot  back  to  their  Arctic  home ; 
but  some  followed  arctic  conditions  upward  to  the  tops  of 
high  mountains,  and  were  stranded  there  in  alpine  isola- 
tion till  now.  It  is  true  they  have  been  slowly  changing 
since  then — some  in  one  direction,  some  in  another — in 
accordance  with  a  universal  law  in  the  case  of  isolated 
species ;  but  the  time  has  been  too  short  to  effect  any 
great  changes. 

2.  South  Africa. — Africa,  south  of  Sahara,  is  inhabited 
by  two  very  distinct  groups  of  mammals.  The  first  group 
consists  of  small  animals  of  very  low  organization,  such 
as  insectivores,  but  very  different  from  those  found  any- 
where else.  These  we  shall  call  indigenes.  The  other 
group  consists  of  very  large  and  highly  organized  ani- 
mals, mostly  also  peculiar  to  Africa,  but  similar  in  gen- 
eral character  to  those  found  in  Eurasia,  especially  those 
of  Pliocene  times.  These  we  shall  call  invaders.  Now, 
before  glacial  times,  Africa  was  isolated  from  the  rest  of 
the  world  and  inhabited  by  the  indigenes  only.  Then 
came  the  glacial  elevation,  opening  gateways  through  the 
Mediterranean  and  into  Africa,  and  the  glacial  cold 
driving  the  Pliocene  mammals  southward  into  Africa, 
where  they  were  shut  up  by  the  closing  of  the  passages 
through  the  Mediterranean  and  by  the  formation  of  the 
Desert  of  Sahara.  The  subsequent  struggles  between 
invaders  and  indigenes,  and  the  effect  of  a  new  environ- 
ment on  the  invaders,  have  greatly  changed  both,  but 


CENOZOIC  ERA.—AOE  OF  MAMMALS.  405 

especially  the  weaker  indigenes.     Thus  have  resulted  the 
mammalian  fauna  of  Africa. 

3.  The  British  Isles. — The  fauna  and  flora  of  the 
British  Isles  are  almost  identical  with  those  of  Europe, 
but  not  quite.  They  differ  in  two  respects,  a.  There 
are  varietal,  though  perhaps  not  specific,  differences  of 
form.  J.  The  number  of  species  is  much  less  than  on 
the  continent.  This  is  especially  true  of  Ireland.  Thus, 
of  90  species  of  European  mammals  only  40  are  found  in 
England  and  22  in  Ireland.  Of  22  European  species  of 
reptiles  and  amphibians  only  13  are  found  in  England 
and  4  in  Ireland.  The  migrations  of  glacial  times  com- 
pletely explain  this.  Before  glacial  times  Great  Britain 
was  a  part  of  Europe  and  had  the  same  fauna  and  flora. 
During  the  glacial  times  it  was  covered  with  the  ice-sheet, 
and  all  life  destroyed  or  driven  southward.  After  the 
glacial  times  it  was  still  connected  with  the  continent, 
and  began  to  be  recolonized  by  migration  from  Europe. 
But  before  the  colonization  was  completed,  especially  for 
more  distant  Ireland,  it  was  again  separated  by  subsidence 
from  the  continent,  and  at  the  same  time  Ireland  was 
separated  from  England.  The  time  since  has  not  been 
sufficient  to  make  species,  although  it  has  been  enough 
to  make  incipient  species,  i.  e.,  geographical  varieties. 

4.  Coast  Islands  of  California. — The  flora  of  Cali- 
fornia consists  of  two  groups  of  species,  the  one  charac- 
teristically Calif ornian,  the  other  more  widely  diffused. 
The  first  is  undoubtedly  indigenous,  the  second  is  prob- 
ably composed  of  invaders  from  the  north.  Now,  off  the 
coast  of  Southern  California  there  is  a  string  of  bold, 
rocky  islands  about  2,000  feet  high  and  separated  from 
the  mainland  by  a  deep  channel  50  miles  wide.  The  flora 
of  these  islands  is  very  peculiar.  Of  300  known  species 
about  50  are  wholly  peculiar  to  the  islands  and  not  found 
elsewhere  in  the  world.  The  remaining  250  are  all 
characteristic  California  species.    Now  the  explanation. 


406  HISTORICAL   GEOLOOY. 

Before  and  during  the  early  part  of  the  Glacial  epoch 
the  islands  were  a  part  of  the  continent.  We  have  already 
given  proof  of  this  on  page  393.  At  that  time  all  was 
inhabited  by  the  same  flora,  viz.,  the  indigenous.  Before 
the  invasion  from  the  north  the  islands  were  separated 
from  the  mainland.  Then  came  the  northern  invasion 
and  consequent  struggle  between  native  and  invading 
species — the  destruction  of  some  natives  and  the  modifi- 
cation of  others —  and  the  final  result  was  the  California 
flora  as  we  now  know  it.  But  the  island  flora  was  spared 
this  conflict,  and  therefore  retained  more  nearly  the  orig- 
inal character  of  both.  In  the  flora  of  these  islands, 
therefore,  we  see  a  near  approach  to  the  flora  of  both 
mainland  and  islands  before  the  separation. 


CHAPTER  VI. 

PSYCHOZOIO  EKA. — AGE  OF  MAN". 

In  all  previous  ages  there  ruled  brute  force  and  ferocity. 
In  this  age  alone  Reason  appears  as  ruler.  The  order  of 
Nature  must  be  adjusted  to  this  keynote.  Therefore,  the 
great  ruling  mammals  of  the  previous  age  must  become 
extinct,  and  the  mammalian  class  must  become  subordi- 
nate ;  noxious  animals  and  plants  must  diminish,  and 
useful  ones  be  preserved. 

Although  in  length  of  time  this  is  not  to  be  compared 
to  an  era,  nor  to  an  age,  nor  to  a  period,  nor  even  to  an 
epoch,  yet  it  deserves  to  be  made  one  of  the  primary 
divisions  of  time,  not  only  on  account  of  the  dignity  of 
man,  but  also,  and  mainly,  because  through  his  agency 
there  is  now  going  on  in  organic  forms  a  change  as  sweep- 
ing as  any  which  has  ever  taken  place.  This  change  has 
been  going  on  ever  since  the  introduction  of  man,  and  is 
going  on  now,  but  will  not  be  complete  until  civilized 
man  occupies  the  whole  earth. 

It  is  interesting  to  mark  some  of  the  steps  of  this 
change.  The  disappearance  of  the  mammoth,  the  mas- 
todon, the  cave-bear,  and  the  saber-toothed  tiger  was 
due,  partly  at  least,  to  man.  These  are  among  the  first. 
Some  of  the  gigantic  oxen  of  Europe  (urus)  lingered 
until  Roman  times.  One  species  (aurochs)  still  lingers, 
being  preserved  by  royal  edict  in  the  forests  of  Lithuania. 
The  bison  or  buffalo  of  our  Western  plains  is  doomed  to 
speedy  extinction,  unless  saved  by  domestication.  In  fact, 
407 


408 


HISTORICAL   GEOLOGY. 


nearly  all   our  domesticated  animals  and  useful  plants 
have  been  thus  saved. 

A  remarkable  example  of  recent  extinction  of  the  Qua- 
ternary species  is  found  in  the   gigantic 
wingless  birds  of  New  Zealand  and  Mada- 
gascar.   The  bones  of  the  Dinornis  and  the 
Epiornis  are  very  abundant  in  these  islands. 
The  Dinornis  giganteus   (Fig.    357)  was 
twelve  feet  high.     The  drumstick  was  a 
yard  long,  and  as  big  as  the  leg- 
bone  of  a  horse.     A  perfect  Qg^ 
of  the  Epiornis  has 
been  found,  six  times 
as  big  as  the  ^gg  of 
an  ostrich.     The  ex- 
tinction    of      these 
birds,     although     it 
occurred   before  the 
discovery    of     these 
islands    by   civilized 
man,   was  so   recent 
that   the    feet    have 
been      found      with 
dried       skin      upon 
them,  and  eggs  with 
the       skeletons      of 
chicks  within. 

Now,  in  this 
gradual  change  from 
the  Quaternary  to 
the  present  fauna 
and  flora,  when  did  man  first  appear  upon  the  scene  and 
become  an  agent  of  change  ?  And  what  kind  of  man 
was  this  primeval  man?  These  are  questions  of  tran- 
scendent importance. 


Fig  357.— Dinornis  giganteus,  x  ^.  (From  a  pho- 
tograph of  a  skeleton  in  Christchurch  Museum, 
New  Zealand.) 


PSYCHOZOIG  ERA.— AGE  OF  MAN.  409 


Antiquity  of  Man, 

On  this  important  question,  history,  archaeology,  and 
geology  meet  and  cooperate  ;  and  it  is  to  the  introduc- 
tion of  geological  methods  that  we  must  attribute  the 
rapid  advances  in  recent  times. 

Archaeologists  long  ago  divided  the  history  of  human 
progress,  according  to  the  nature  of  the  implements  used, 
into  three  ages — a  stone  age,  a  bronze  age,  and  an  iron 
age.  Again,  by  closer  study,  they  subdivided  the  stone 
age  into  an  older  stone  {Paleolithic)  and  a  newer  stone 
(Neolithic)  age.  In  the  one,  the  stone  implements  are 
chipped  ;  in  the  other,  polished.  Again,  under  the  guid- 
ance of  geology,  the  Paleolithic  has  been  subdivided  into 
the  mammoth  age  and  the  reindeer  age.  In  the  former, 
man  was  contemporaneous  with  the  mammoth,  the  cave- 
bear,  and  other  extinct  Quaternary  animals  ;  in  the  latter, 
the  mammoth  had  nearly  disappeared,  but  the  reindeer 
was  abundant  over  all  middle  and  southern  Europe.  The 
flint  implements  in  the  former  were  so  rude  that  they 
might  well  be  called  flint-flakes  ;  in  the  latter  they  were 
carefully  chipped.  The  former  was  coincident  with  the 
Mid-Quaternary,  i.  e.,  Champlain,  oriperh.a])sInterr/lacial; 
the  latter  with  the  second  Glacial  or  the  early  Post-glacial. 


3.  Iron  age 

2.  Bronze  age \-  Psychozoic. 

!  Neolithic — Domestic  animals. 
^  Reindeer — Late  Quaternary. 
Paleolithic 

Mammoth — Mid-Quaternary. 


a 


As  seen  by  the  schedule  above,  the  Psychozoic  e'ra  and 
age  of  man  commences  with  the  Neolithic.  Before  that 
time,  man  existed,  indeed,  but  contended  doubtfully  for 
mastery  with  the  great  Quaternary  animals.  From  that 
time  his  victory  is  assured  and  his  reign  begins. 


410  HISTORICAL   GEOLOGY, 

Primeval  Man  in  Europe. 

According  to  our  schedule,  man  is  traced  back  to  the 
Mid-Quaternary.  Some  geologists  think  that  there  are 
signs  of  his  existence  still  earlier,  viz.,  in  the  Tertiary  ; 
but  the  evidence  is  acknowledged  to  be  unsatisfactory. 
We  shall  confine  ourselves,  therefore,  to  Quaternary  man. 
We  shall  commence  with  Europe,  as  the  evidence  is  more 
complete,  and  all  the  steps  represented. 

Quaternary  Man ;  Mammoth  Age ;  the  River- 
Drift  Man. — Some  twenty  years  ago,  M.  Boucher  de 
Perthes  found,  in  the  undisturbed  gravels  of  the  upper 
terraces  of  the  river  Somme,  the  implements  of  man  asso- 
ciated with  the  bones  of  many  extinct  Quaternary  animals, 
such  as  the  mammoth,  the  rhinoceros,  the  hippopotamus, 
the  hyena,  the  horse,  the  Irish  elk,  the  cave-lion,  etc. 
The  doubts  which  were  at  first  entertained  by  the  more 
cautious  geologists  have  been  entirely  removed  by  careful 
examination.  We  give  this  as  only  one  example  of  very 
many.  In  all  cases  the  implements  are  of  the  rudest  kind 
of  flaked  flints,  like  those  figured  on  page  414. 

The  Cave -Man. — In  Quaternary  times,  man  un- 
doubtedly contested  with  the  hyena,  the  lion,  the  saber- 
toothed  tiger,  and  the  cave-bear  the  right  to  occupy  the 
caves  as  homes.  The  evidence  of  this  is  found  in  the 
association  of  his  implements,  and  even  his  bones,  with 
those  of  all  the  extinct  carnivores  mentioned,  under  con- 
ditions which  admit  of  no  doubt  of  their  contempo- 
raneousness. They  are  sometimes  entombed  together, 
and  covered  with  stalagmitic  crust,  which  has  never  been 
broken  from  Quaternary  times  until  rifled  by  the  geolo- 
gist.    We  give  a  single  example. 

The  Mentone  Man. — In  a  cave  at  Mentone,  near 
Nice,  has  been  recently  found  the  almost  perfect  skeleton 
of  an  old  man,  of  more  than  average  height,  lying  on 
his  side  in  an  easy  position,  and  about  him  chipped  im- 


PSYCHO  ZOIC  ERA.— AGE  OF  MAN. 


411 


plements  and  bones  of  extinct  animals,  among  which 
were  many  pierced  reindeer^s  teeth.  All  of  these  were 
perfectly  preserved  by  a  stalagmitic  crust.  We  may  well 
imagine  that  this  old  hunter,  finding  his  end  approaching, 
retired  to  his  cave-home,  laid  himself  quietly  down,  with 
the  implements  and  trophies  of  successful  chase  about 
him,  and  gave  up  the  ghost.  Good  Mother  Nature  then 
slowly  buried  his  remains,  and  sealed  them  up  beneath  a 
crust  of  stalagmite. 

The  Primeval  Aquitanians. — In  southwestern 
France,  on  the  river  Vizere,  a  branch  of  the  Dordogne, 
are  found  many  caves  which  were  inhabited  by  a  more 
peaceful  race.  They  were  not  only  hunters,  but  also 
fishers  ;  for  we  find,  besides  stone  implements,  many  im- 
plements made  of  bone,  among  which  are  rude  fishhooks. 
They  also  show  evidence  of  some  skill  in  drawing  and 
carving.  Among  the  bone  implements  found  there  are 
many  drawings  of  extinct  animals.  Fig.  358  represents 
a   rude  but    very  characteristic   sketch  of  a  mammoth. 


Fig.  358. — Drawing  of  a  Mammoth  by  contemporaneous  man. 

made  by  contemporaneous  man.  In  these  caves  we  find 
a  gradual  transition  from  the  mammoth  to  the  reindeer 
age. 

General  Conclusions.— These  all  belong  to  the  Qua- 
ternary.    In  Europe,  therefore,  man   certainly   saw  the 


412  HISTORICAL    GUOLOGY. 

flooded  rivers  and  lakes,  and  probably  the  great  glaciers. 
He  certainly  hunted  the  great  extinct  Quaternary  animals, 
the  mammoth,  the  cave-bear,  the  cave-lion,  the  great  Irish 
elk,  and  the  reindeer.  All  the  evidence  points  to  an  ex- 
tremely low,  savage  state,  with  little  or  no  tribal  organi- 
zation. There  is  no  evidence  yet  of  either  domestic  ani- 
mals or  of  agriculture. 

Neolithic  Man. 

Kitchen-Miclclens  ;   Refuse-Heaps  ;  Shell-Mounds. 

— In  many  parts  of  Europe,  especially  in  Denmark  and 
Sweden,  are  found  mounds,  composed  wholly  of  shells 
and  other  refuse  of  tribal  gatherings  and  feastings.  The 
men  of  that  time  seem  to  have  had  the  habit  of  gathering 
annually  at  some  place  where  food  was  abundant,  usually 
on  the  seashore,  at  the  mouth  of  a  river.  From  year  to 
year  the  refuse  of  such  gatherings  accumulated  until 
mounds  of  great  extent  were  gradually  formed.  In  these 
mounds  are  found  the  bones  of  men  and  animals  and  the 
implements  of  men,  and  from  these  we  may  form  a  good 
idea  of  the  character  and  habits  of  the  men. 

Here,  then,  we  find  a  great  and  somewhat  sudden 
change  :  1.  There  are  no  longer  any  extinct  Quaternary 
animals.  2.  We  find  here,  for  the  first  time,  domestic 
animals,  viz.,  the  dog,  the  ox,  the  sheep,  etc.,  and  also 
evidences  of  agriculture.  3.  The  implements  are  no 
longer  only  chipped,  but  are  often  carefully  polished  by 
rubbing.  Rude  pottery  is  also  found.  4.  We  have  here 
for  the  first  time  the  evidence  of  tribal  organization, 
similar  to  the  savage  races  of  the  present  day,  5.  The 
conformation  of  the  skull  shows  a  different  race  from  that 
of  the  cave  and  river-drift  men.  In  a  word,  we  have  here 
the  appearance  in  Europe,  probably  by  migration,  of 
a  different  and  higher  race.  Until  this  time  man  in 
Europe  seems  to  have  contended  doubtfully  with  wild 
animals :    now    he    seems    to   have  established    his   su- 


\ 


PSYCHOZOIG  ERA.— AGE  OF  MAN, 


413 


premacy.  The  Psychozoic  era  and  age  of  man,  there- 
fore, rightly  commence  here,  and  all  that  follows  may 
be  claimed  by  archaeology  and  history.  Nevertheless, 
we  shall  give  a  very  brief  sketch  of  further  progress. 

Transition  to  the  Bronze  Age. 

L.ake-Dwellers. — In  1850  the  lakes  of  Switzerland  be- 
came very  low,  and  a  great  number  of  wooden  piles  were 
exposed.  Interest  being  excited,  the  same  was  found  to 
be  true  of  all  the  lakes  of  middle  Europe.  By  dredging, 
implements  of  war,  of  the  chase,  of  husbandry,  and  orna- 
ments and  trinkets  of  all  kinds  were  found  in  great  abun- 
dance. Some  of  these  were  polished  stone,  but  most  were 
bronze,  and  often  beautifully  finished.  Remnants  of  grain 
and  fruits  of  several  kinds  were  also  found.  From  these 
findings  the  houses  (Fig.  359),  the  habits,  and  the  mode 


Fig.  359.— Lake-dwellings,  restored.    (After  Mortillet) 

of  life  of  this  people  have  been  reconstructed,  and  even 
a  novel  embodying  their  life  has  been  written.* 

Thus  we  might  continue,  by  means  of  remains  alone, 
to  trace  progress,  through  Roman  graves,  Roman  roads 
*  **  ftealmax,"  by  Arthur  Helps. 


414  HISTORICAL   GEOLOGY. 

and  implements,  etc.,  to  the  graves  in  our  own  church- 
yards and  the  machinery  of  our  own  times.  This  all  be- 
longs to  history.  Thus  we  trace  geology  into  archaeology, 
and  archaeology  into  history. 

Primeval  Man  in  America. 

It  must  be  remembered  that  the  different  men  we  have 
/described  in  Europe  represent  different  stages  of  progress 
there.  The  progress  has  not  been  at  the  same  rate  every- 
where, and  therefore  the  different  stages  are  not  necessa- 
rily contemporaneous.  When  America  was  discovered, 
the  native  tribes  were  still  in  the  stone  age,  and  many 
savages  are  only  in  this  stage  of  advance  now.  The 
advance  was  more  rapid  in  Europe,  apparently  because 
of  the  frequent  and  extensive  migrations  and  conflict  of 
races  there.  Nevertheless,  the  rudest  state  (Paleolithic 
age)  seems  to  haye  oeen  nearly  contemporaneous  in 
America  and  Europe,  and  probably  elsewhere. 

Quaternary  River-Drift  Man  in  America. — There 
are  many  examples  of  rude  flint-Hakes  in  the  i-iver-gravels 
of  California  and  in  the  glacial  drift  oi  New  Jersey  and 
Ohio.     These  were,  it  is  believed,  the  work  of  a  race  cor- 


PiQ.  360.— Paleolith  found  by  Abbott  in   New  Jersey,  slightly  reduced,     (After 

Wright.) 


PSYCHO  ZOIC  ERA.— AGE  OF  MAN.  415 

responding  to  and  contemporaneous  with  the  river-drift 
man  of  Europe  (Fig.  360).  Some  doubts  have  been 
recently  thrown  on  the  antiquity  of  these  findings.  For 
this  reason  we  will  not  dwell  on  Glacial  man  in  America. 

Neolithic  Man  in  America. — The  Neolithic  age  is 
represented  here,  as  in  Europe,  by  refuse-heaps,  which 
were  evidently  made  in  the  same  way  as  those  already 
described,  and  have  similar  contents.  They  are  abun- 
dant on  the  seacoasts  everywhere,  and  some  of  them  are 
probably  no  older  than  the  discovery  of  America ;  for, 
as  already  said,  the  native  tribes  were  then  still  in  the 
stone  age. 

Moiind-Builders. — The  bronze  age  is  probably, 
though  imperfectly,  represented  by  the  mound-builders. 
In  many  places,  especially  in  the  valley  of  the  Mississippi, 
are  found  mounds  of  enormous  size,  and  fortifications 
and  communal  houses  of  somewhat  elaborate  construc- 
tion. In  connection  with  these  have  also  been  found  not 
only  highly  polished  stone  implements,  but  also  imple- 
ments of  hammered  copper.  The  copper-mines  of  Lake 
Superior  were  evidently  worked  by  them,  as  the  old  work- 
ings have  been  found.  The  mound-builders  were  prob- 
ably a  different  race  from  the  hunter  tribes  of  Indians, 
and  more  advanced,  although  many  now  think  they  are 
the  same. 

Cliff-Dwellers. — In  the  dry  regions  of  New  Mexico 
and  Arizona  the  almost  perpendicular  cliffs  bordering 
the  mesas  are  studded  with  remains  of  many-storied  com- 
munal houses  of  stone.  There  are  small  remnants  of  sev- 
eral tribes  in  that  region — Pueblos,  Moquis,  and  Zunis — 
that  live  now  in  similar  dwellings,  on  the  flat  tops  of 
almost  inaccessible  mesas.  One  dwelling  with  many 
rooms  is  occupied  by  a  whole  community.  These  also  are 
entirely  different  from  the  roving  tribes,  and  by  many 
are  connected  with  the  Aztecs  on  the  one  hand,  and  the 
mound-builders  on  the  other. 


'416  HISTORICAL  GEOLOGY. 

It  is  needless  to  repeat  that  these  last  three  heads  be- 
long to  the  present  epoch. 

Conclusions, 

1.  "We  have  thus  traced  man  back  to  the  Mid-Quater- 
nary. It  is  possible  that  he  may  hereafter  be  traced  still 
further  back  ;  but  this  seems  very  improbable.  No  mam- 
malian species  now  living  can  be  traced  further  back  than 
the  Quaternary.  Man  belongs  to  the  present  mamma- 
lian fauna,  and  probably  came  in  with  other  mammalian 
species  in  the  Quaternary. 

2.  We  have  not  yet  been  able  to  find  any  undoubted 
transition  forms  or  connecting  links  between  man  and  the 
highest  animals.*  The  earliest  known  man,  the  river- 
drift  man,  though  in  a  low  state  of  civilization,  was  as 
thoroughly  human  as  any  of  us. 

3.  The  amount  of  time  which  has  elapsed  since  man 
first  appeared  is  still  doubtful.  Some  estimate  it  at  more 
than  a  hundred  thousand  years — some  only  ten  thousand. 
The  question  should  not  be  regarded  as  of  any  impor- 
tance, except  as  a  question  of  science. 

*  Such  a  link  is  supposed,  by  many,  to  have  been  recently  found 
in  Java,  and  named  Pithecanthropus.     We  wait  for  more  evidence. 


INDEX, 


Acrogens  (point-growers  or  apex- 
growers),  age  of,  295. 

African  fauna  explained,  404. 

Agencies,  geological,  9. 
leveling  and  elevating,  131. 

Ages,  360. 

Air,  chemical  action  of,  14. 
mechanical  action  of,  15. 

Alberrite;  a  form  of  asphalt,  313 

Alkaline  lakes,  deposits  in,  77. 

Alpine  species,  403. 

Ammonite  (horn  of  amnion  stone), 
330. 

Amphibians  (living  in  both   [air 
and  water] ), called  also  batra- 
chians,  319-328. 
age  of,  297. 

AraphicQjlias  {amphi^  both  sides; 
koilos,  hollow  ;  double  con- 
cave), 347. 

Amygdaloid,  223. 

Anchisaurus,  344. 

Ancyloceras  (curved  horn),  353. 

Andesite,  214. 

Angustifolius  (narrow  leaf),  274. 

Anomodont  (lawless  tooth),  328. 

Anomcepus  (unlike  feet),  343 

Anoplotherium  (unarmed  beast), 
380. 

Anticline  and  syncUne  defined, 
188. 

Apiocrinus  (pear  crinoid),  331. 

Appalachian  revolution,  322. 

Aqueous  agencies,  17. 

Aquitanians,  primeval,  411. 

Archa3an     (relating    to    earliest 
times),  259. 
rocks,   area  of,   in  the  United 

States,  265. 
rocks,  character  of,  264. 

Le  Conte,  Geol.  ^ 


Archaean  system,  263. 
times,  life  of,  265. 
times,   physical  geography  ol, 
265. 

Archa?ozoic  (primeval  life),  259. 

Archegosaurus    (primordial    liz- 
ard), 320. 

ArchaBopteryx  (primordial  winged 
creature),  337. 

Artesian  wells,  70. 

Asteroid  (star-like),  277. 

Asterolepis  (star-scale),  291. 

Atlantosaur  (great  lizard)   beds, 
345. 

Atmosphere,  chemical  action  of, 
14. 
mechanical  action  of,  15. 

Atmospheric  agencies,  10. 

Baculite  (stone-staff),  364 
Bad  Lands,  248,  366. 
Banks,  Bahama,  50. 

in  North  Sea,  50. 

of  Newfoundland,  50. 

submarine,  49. 
Bars,  how  formed,  38. 

position  of,  38. 

removal  of,  40. 
Basalt,  139,  215. 

columnar  structure  of,  220 
Base  level  of  erosion,  28. 
Bed-rock  surface,  387. 
Belemnite  (stone  dart),  332. 
Birds,  337. 

Bitumen  and  petroleum,  318. 
Blastids  (bud-like),  279. 
Borax  lakes,  77. 
Botanical  regions,  119. 
Bowlders,  387. 

of  disintegration,  12. 

417 


418 


INDEX. 


Brachiopod  (arm-foot),  279. 

Breccia,  180. 
volcanic,  222. 

British  Isles,  fauna  of,  405. 

Brontosaur  (giant  lizard),  345. 

Brontotherium  (giant  beast),  378. 

Brontozoura  (giant  animal),  343. 

Bronze  age,  409,  413. 

Bryozo6n(nioss-aniraal,  called  also 
Polyzoon:  an  order  of  com- 
pound moUuscoid  animals), 
316. 

Buthotrephis  (reared  in  the  deep), 
274. 

Butterfly,  a  fossil,  372. 

Calaraite  (stone  reed:  a  family  of 
coal-plants  allied  to  equisetae), 
309. 
California  coast  isles,  405. 
Cambrian,  271. 
CaSons,  ravines,  gorges,  23. 

examples  of,  24. 
Carboniferous  (coal-bearing)  age, 
298. 
age,  fauna  of,  315. 
age,  subdivisions  of,  297. 
Caves,  limestone,  71. 
Cenozoic  (pertaining  to  recent  ani- 
mal life)  era,  363. 
era,  characteristics  of,  363. 
era,  subdivisions  of,  364. 
Cephalaspis  (head-shield),  292. 
Cephalopod     (head-foot  :     refer- 
ring to  position  of    limbs), 
281. 
Ceratite  (stone-horn:  a  family  of 

shelled  cephalopods),  327. 
Ceratodus  (horn-tooth),  295. 
Cestracion  (sharp  tool:  referring 

to  the  spine),  296. 
Chalk,  350. 

Champlain  epoch,  390. 
Chemical  deposits  in  lakes,  76. 
deposits  in  springs,  72. 
deposits  of  iron  oxide,  75. 
deposits  of  lime  carbonate,  73. 
"  deposits  of  silica,  76. 

deposits  of  sulphur,  76. 
Chronology,  construction  of  geo- 
logical 207. 
Cinders,  ashes,  etc.,  135. 
Cleavage,  slaty,  194. 


Cleavage,  structure,  193. 
Clilf-dwellers,  415. 
Coal-fields  of  the  United  States, 
300. 
of  Eastern  Virginia  and  North 
Carolina,  344. 
Coal  measures,  298. 

mode  of  accumulation  of,  310. 
origin  of,  801. 
period,  climate  of,  312. 
period,  length  of,  303. 
period,  physical  geography  of, 

312. 
plants  of  the,  303. 
varieties  of,  301. 
Coccosteus  (berry-bone),  292. 
Colorado  Canon,  25. 
Columbia  River  and  tributaries, 

22. 
Columnaria     alveolata     (cellular 

columns),  276. 
Columnar  structure,  220. 

structure,  cause  of,  221. 
Compsognathus  (handsome  jaw). 

337. 
Concretionary  or  nodular   struc 

ture,  198. 
Concretions,  how  formed,  199. 
Conformity    and    unconformity, 

190. 
Conglomerate,  volcanic,  222. 
Connecticut  River  Valley  tracks, 

342. 
Continental  faunas  and  floras,  123. 
faunas  and  floras,  subdivisiont 

of,  126. 
form,  general  laws  of,  176. 
Continents  and  sea-bottoms,  ori- 
gin of,  178. 
mean  height  of,  176. 
Coral,      compound,      mode      of 
growth,  95. 
conditions  of  growth,  98. 
forest,  how  formed,  96. 
islands,  closed  lagoons,  103. 
islands,  how  formed,  97. 
islands,  lagoonless,  103. 
islands  of  the  Pacific,  98. 
polyp,  structure  of,  93. 
reef-rock,  97. 

reef- rock, different  kinds  of,  108. 
reefs,  97. 
reefs,  atolls,  103. 


INDEX, 


419 


Coral  reefs,  barrier,  101. 
reefs,  fringing,  100. 
reefs,  how  formed,  97. 
reefs  of  Florida,  109. 
reefs  of  the  Pacific,  99. 
reefs,  theories  of  barriers  and 
atolls,  103. 
Corals  in  Paleozoic  rocks,  275, 
Cordaites  (a    coal-plant    named 

after  Corda),  304. 
Coryphodon  (peak-tooth),  377. 
Co-seismal  lines  (lines  connecting 
points  which  feel  a  shock  at 
the  same  moment),  103. 
Crater  lake,  142. 
Cretaceous  period,  348. 
period,  animals  of,  352. 
period,  areas  of  rocks  of,  348. 
period,  coal  of,  351. 
period,  physical  geography  of, 

348. 
period,  plants  of,  351. 
Crinoid  (lilylike  stone),  277. 

range  in  time,  278. 
Crust  of  the  earth,  174. 

general  configuration  of,  176. 
Cyathophylloid  (cup-leaf like), 

275. 
Cycads  :  plants  of  cycas,  or  sago- 
palm  family,  329. 
Cystid  (baglike),  279. 

Darwin's    subsidence    theory    of 

atolls,  104. 
Deltas,  how  formed,  33. 

age  of,  36. 

subsidence  of,  169. 
Dendrerpeton  (tree-reptile),  320. 
Denudation,  or  general   erosion, 
252. 

modes  of  determining  amount 
of,  253. 
Deposits,  chemical,  72,  182. 

deep-sea,  117. 

made  by  waves,  50. 

mechanical,  182. 

organic,  182. 
Devonian  age,  286. 

age,  animals  of,  288. 

age,  fishes  of,  291. 

age,  life-system  of,  287. 

age,  physical  geography  of,  287. 

age,  plants  of,  286. 


Devonian  age,  rocks  of,  286. 

Devonian  fishes,  sudden  appear- 
ance of,  297. 

Diatoms  (5raroyUo?,  cut  in  two)  : 
microscopic      plants     which 
multiply  by  dividing  in  two, 
115,  352. 
shell  deposits  of,  116. 

Diabase,  212,  213. 

Dicotyls.  contraction  for  dicoty- 
ledons :  plants  having  two 
seed-leaves,  351. 

Dicynodon  (two  canine-toothed), 
327. 

Dikes,  141,  216. 
effect  on  stratified  rocks,  217. 

Dinichthys  (huge  fish),  293. 

Dinoceras  (huge  horned  animal), 
377. 

Dinosaur  (huge  lizard),  334. 

Dinotherium  (huge  beast),  381. 

Diorite,  217. 

Dip  and  strike  defined,  187. 

Diplacanthus  (double  spine),  293. 

Diplocynodon  (double  canine- 
teeth),  347. 

Diprotodon  (two  front  teeth),  402. 

Disintegration,  rate  of,  13. 

Dolerite,  214. 

Drift,  386. 

Drift-timber,  88. 

Dromatherium  (running  beast), 
345. 

Dynamical  geology,  9. 

Earth,  crust  of,  174. 

crust,  cause  of  inequalities  in, 

178-240. 
crust,  cause   of  movement  of, 

171. 
crust,   gradual    oscillation    of. 

164. 
density  of,  174. 
general  form  of,  173. 
general  structure  of,  173. 
general  structure  of,  means  of 

observing,  175. 
internal  heat  of,  131. 
Earthquake,  epicentrum  of,  156. 
focus,    mode   of    determining, 

163. 
wave,  nature  of,  158. 
wave,  velocity  of,  156. 


420 


INDEX. 


Earthquakes,  154. 
beneath  the  sea-bed,  159. 
cause  of,  157. 

connection  of,  with  phases  of  the 
moon,  and  with  the  weather, 
163. 
frequency  of,  155. 
great  sea-wave  of,  160. 
phenomena  of,  155. 
Echinoderm  (spiny  skin),  277. 
Echinoid  (urchin-like),  277,  352. 
Echinus  (hedgehog  or  urchin)  :  a 

sea-urchin,  277. 
Elasmobranchs,  291. 
Elevation  and  subsidence,  cause 
of,  171. 
of  crust,  gradual,  165. 
Eocene  (dawn  of  recency),  864. 
Eohippus(dawn  of  earliest  horsey, 

377. 
EozoCn  (dawn  animal),  266. 
Epicentrum  (upon  the  center),  156. 
of  an  earthquake,  mode  of  de- 
termining, 163. 
Epochs,  262. 
]^]quiseta3 :    horse-tails,    scouring- 

rush,  287,  306,  326. 
Eras,  259. 

Erosion,  agents  of,  252. 
amount  of,  253. 
average  rate  of,  19. 
general,  or  denudation,  252. 
general  results  of  glacial,  390. 
of  rain  and  rivers,  18. 
Eruptive  rocks,  true,  214. 
Estuaries,  deposits  in,  38. 

how  formed,  37. 
Evolution,   bearing  of  Devonian 
fishes  on,  295. 

Falls,  Niagara,  20. 

of  St.  Anthony,  22. 

Minnehaha,  22. 

Yosemite,  23. 
False  bedding,  184. 
Faults,  amount  of  displacement, 
230. 

kinds  of,  232. 

law  of  slip,  232. 
Faunas   and   floras,   continental, 
123 

(bfined,  118. 

marine,  129. 


Faunas  and  floras,  geographical, 

explained,  403. 
Favositid  (honeycomblike  stone), 

275. 
Felsite,  213. 
Ferns,  306. 

Fiords  (Norwegian  term  for  deep 
inlets    between    high    head- 
lands), 45. 
origin  of,  396. 
Fishes,  age  of,  286. 
Fissures,  great,  229. 

great  characteristics  of,  230 
Flood-plain,  30. 
of  the  Mississippi,  31. 
of  the  Nile,  age  of,  31. 
Floras,  defined,  118. 
Florida,  reefs  and  keys  of,  109. 
Floriformis  (flower-like),  275. 
Folded  strata,  186. 
Foraminifera  (full  of  holes  :  pro- 
tozoan   animals   with  perfo- 
rated shells),  351. 
Formation  defined,  204. 

geological,  193. 
Fossils,  200. 

degrees  of  preservation  of,  200. 
Fossil  species,  distribution  of,  203. 
Frost,  action  of,  in  soil-making, 

15. 
Fucoid  (resembling  tangle),  274. 
Fucus  (tangle  or  wrack),  274. 
Fumaroles  (smoking  vents),  145. 

Gabbro,  212. 

Ganoid  (shining  :  referring  to  the 

scales),  291. 
Gasteropod  (belly-foot  :  referring 

to  mode  of  walking),  280. 
Gastornis  (Gaston's  bird),  374. 
Geographical  distribution  of  spe- 
cies, 118. 
diversity  of  species,  origin  of, 
130. 
Geological   and  humai^    history, 
correspondence  of  great  prin- 
ciples of,  256. 
chronology,     construction    of, 

207. 
formation,  193. 
history,  divisions  of,  359. 
Geolo^^j^y,  definition  of,  7. 
dynamical,  9. 


I 


INDEX, 


421 


Geology,  great  divisions  of,  8. 
historical,  256. 
structural,  173. 
Geyser,  Great,  147. 

Great,  phenomena  of  eruption 
of,  147. 
Geysers  defined,  146. 

cause  of  eruption  of,  151. 
of  Iceland,  146. 
of  Yellowstone  Park,  147. 
Gigantitherium  (gigantic   beast), 

343. 
Glacial  (icy),  385. 
cold,  cause  of,  402. 
epoch,  386. 

epoch,  explanation  of  phenom- 
ena of,  358. 
erosion,  general  results  of,  396. 
Glaciers  as  a  geological  agent,  60. 
characteristic  signs  of,  02. 
defined,  52. 
erosion  of,  61. 

evidences  of  former  greater  ex- 
tension of,  62. 
in  the  Sierra,  56. 
lower  limit  of,  53. 
motion  of,  58. 

size  of,  in  various  regions,  56. 
structure  of,  56. 
transportation  and  deposit  by, 
01,  62. 
Globigerina  (globule-bearing), 117. 

ooze,  117. 

Glyptodon  (sculptured  tooth),  400. 

Goniatite  (angled  stone  :  a  family 

of  shelled  cephalopods  with 

angled  sutures),  290,  317. 

Gorge  formed  by  recession  of  falls, 

21. 
Gracilis  (graceful),  274. 
Gradual  oscillation  of  the  earth. 

crust,  164. 
Grahamite  :    a  form   of  asphalt, 

313. 
Granite,  212. 

Granitic  rocks,     composition  of, 
211. 
rocks,  mode  of  occurrence,  212. 
Graphic  granite,  212. 
Graptolites  (stone-writing),  276. 
Ground-water,  perpetual,  08. 
Gulf   Stream,    geological   agency 
of,  48. 


Gulf  Stream,  origin  and  cause  of, 
47. 

Gymnosperm  (naked  seed :  a  class 
of  plants  including  conifers 
and  cycads),  287,  304. 

ITalysites  (stone  chain)  catenulata 

(like  a  little  chain),  276, 
Halysitid  (stone  chain),  275. 
Hamite  (stone  hook),  354. 
Eesperornis  (western  bird),  358, 
Hipparion  (little  horse),  381. 
Hippurite  (horse-tail),  353. 
Historical  geology,  256. 
tlistory,    general    principles   of, 

256. 
geological,  divisions  of,  259. 
Horse,  genesis  of,  382. 
Hybodont  (hybodus,  hump-tooth  : 

a  familv  of  shark-like  fishes), 

318. 
Hydrothermal  fusion  (fusion  by 

heat  and  water),  133. 
Ilydrozoa  (water-animals),  275. 
Hypsilophodon,  355. 

Ice,  agency  of,  52. 

Icebergs  as  a  geological  agent,  66. 

effect   of,  compared  with    gla- 
ciers, 66. 

how  formed,  64. 

of  Greenland,  64. 

of  the  Antarctic,  66. 
Ice-sheet  moraine,  388. 
Ichthyornis  (fish-bird),  358. 
Ichthyosaur  (fish-lizard),  334. 
Ideal  section  of  earth-crust,  261. 
Igneous  agencies,  131. 

rocks,  2l0. 

rocks,  characteristics  of,  210. 

rocks,  classification  of,  211. 

rocks,  extent  of,  on  the  sur/acej 
211. 

rocks,  modes  of  occurrence  of, 
211,  216. 

rocks,  origin  of,  210. 

rocks,  sub-groups  of,  213- 
Iguanodon  (iguana-toothed),  336. 
Indusium(an  inner  garment*  b71. 
Insects,  290. 
Intercalary  beds,  219. 
Invertebrates,  age  of,  271. 
Iron  accumulations,  88. 


422 


INDEX. 


Iron    accumulations,     mode     of 
formation  of,  89. 
bog-ore,  89. 
oxide,  deposits  of,  75. 
Islands :  coast  islands,  how  formed, 
51. 

Joints,  228. 
Jurassic  period,  329. 

period,  animals  of,  329. 

period,  coal  of,  329. 

period,  plants  of,  329. 
Jura-Trias,    disturbances    which 
closed,  347. 

in  America,  341. 

life-system  of,  341. 

Kitchen-middens,  412. 

Labyrinthodont         (labyrinthine 
tooth  :   a    family  of  extinct 
amphibians),  319. 
Lake  Agassiz,  391. 

Bonneville,  394. 

dwellers,  413. 

Lahontan,  395. 

margins,  391. 
Lakes,  alkaline,  77. 

borax,  77. 

chemical  deposits  in,  80. 

erater,  142. 

glacial  origin  of,  396. 

salt,  76. 
Lamellibranch  (plate-gill),  279. 
Lamination,  cross  or  oblique,  184. 
Land,  mean  height  of,  176. 
Laosaurus,  346. 
Laramie  epoch,  360. 

epoch,  coal  of,  361. 
Lava,  classification  of,  139. 

kinds  of,  137. 

sheets,  extent  of,  217. 
Lepidodendrid,  287,  307. 
Lepidodendron  (scale-tree),  307. 
Lepidoganoid  (scale-ganoid),  293. 
Lepidosiren  (scaly  siren  :  an  am- 
phibious fish),  295. 
Levees,  artificial,  32. 

natural,  31. 
Lime  accumulations,  91. 

carbonate,  deposits  of,  73. 

sinks,  72. 
Limestone  caves,  how  formed,  71. 


Limestone  shell,  114. 

Limuloids  :  Limulus  family,  316. 

Limulus ;  horseshoe-crab,  or  king- 
crab,  284,  316. 

Lithodomi  {lithos,  stone,  domus, 
house  :  a  species  of  shell-fish 
which  burrow  in  rocks),  167. 

Lost  intervals  explained,  267. 

Lycopod  (wolfs  foot :  an  order  of 
club-mosses),  307. 

Machairodus  (saber-toothed),  381, 

402. 
Mammals,  age  of,  363. 

genesis  of  orders,  382. 

of  the  Tertiary  period,  374. 
Mammoth,  398. 
Man,  antiquity  of,  409. 

Neolithic,  412. 

of  the  caves,  410. 

of  the  river-drift,  410. 

primeval,  in  America,  414. 

primeval,  in  Europe,  410. 
Marmites  des  geants,  63. 
Mastodon  (nipple-toothed),  399. 
Mastodonsaur    (teat-toothed   liz- 
ard), 327. 
Mauvaises  Terres,  248,  366. 
Megalosaur  (great  lizard),  336. 
Megatherium  (great  beast),  401. 
Mentone  man,  410. 
Mesohippus  (mid-horse),  379. 
Mesozoic   (pertaining   to    middle 
animal  life)  era,  324. 

era,  characteristics  of,  324. 

era,   disturbance  which  closed, 
360. 

era,    general  observations  on, 
359. 

era,  subdivisions  of,  324. 
Metamorphic  rocks,  224. 
Metamorphism,  cause  of,  226. 

agents  of,  226. 
Migrations  during  Glacial  epoch, 

403. 
Mineral  springs,  71. 

veins,  233. 
Miocene  (less  recent),  364. 
Miohippus  (less  horse-like),  378. 
Mississippi  delta,  34. 
Mode  of  accumulation    of  coal, 

310. 
MoUusks,  279. 


INDEX. 


423 


Mono  Lake  in  Quaternary  period, 
394. 

Monotremes  (one. vent  :  the  lowest 
order  of  mammals,  including 
ornithorhynchus    and  echid- 
na), 338. 
Monticles  (little  mountains),  136, 
Moraines,  57. 

Mosasaur  (Meuse  lizard),  357. 
Mound-builders,  415. 
Mountain  life,  different  stages  of, 
245. 
sculpture,  246. 
sculpture  forms  of,  247. 
strata,  thickness  of,  244. 
Mountains,  defined,  238. 

structure  and  origin  of,  238. 
Murray's  theory  of  atolls,  105. 
Myrraecobius  (ant-liver),  340. 

Nautilus,  281. 

Neolithic  (new  stone),  412. 

Niagara  Falls,  recession  of,  21. 

gorge,  origin  of,  21. 
Nodules,  forms  of,  198. 

how  formed,  199. 

Obsidian,  214. 
Ocean,  agency  of,  41. 

mean  depth  of,  176. 

why  is  it  salt  ?  79. 
Oceanic  currents,  47. 
Organic  agencies,  83. 

agencies,  subdivisions  of,  83. 
Orohippus  (mountain-horse),  377. 
Orthoceratite      (straight      stone 

horn),  282. 
Orthoclase  (right  cleavage),  212, 

215. 
Osteolepis  (bony  scale),  294. 
Otozoura  (giant  animal),  342. 
Outcrop,  186. 
Overflows,  217. 

Pacific  bottom ,  subsiding  area,  106. 
Paleolithic  (old  stone),  409. 
Paleotherium  (old  beast),  380. 
Paleozoic   (pertaining   to   old   or 
ancient  animal  life),  259. 
era,    general   observations    on, 

321. 
era,  progressive  changes  during, 
821. 


Paleozoic    era,    subdivisions    of, 
271. 
rocks,  268. 
rocks  and  era,  267. 
rocks,    area  of.    in  the  United 

States,  269. 
system,  unconformity  of,  with 

Archaean,  267. 
times,  growth  of  the  continent 

in,  271. 
times,  physical  geography  of, 
269. 
Peat,  antiseptic  property  of,  85. 
bogs,  84. 

bogs,  rate  of  grovj^th  of,  87. 
bogs,  structure  of,  87. 
composition  of,  84. 
mode  of  accumulation  of,  85. 
swamps,  84. 
Pegmatite,  212. 
Peneplain,  28. 

Period,  geological,  defined,  204. 
Periods  and  epochs,  262. 
Permian  period,  323. 
Petroleum  (rock-oil),  318, 
age  of  strata  of,  314. 
origin  of,  315. 
Phonolite  (ringing-stone),  214. 
Pithecanthropus,  416. 
Placode  rm  (plate-skin),  293. 
Placoganoid  (plate-ganoid),  293. 
Plagioclase    (oblique     cleavage), 

212,  215. 
Plesiosaur    (near    to    a    lizard), 

334. 
Pliocene  (more  recent),  365. 
Pliohippus  (more  horse-like).  379. 
Plumularia  (plume-like),  276. 
Plutonic  rocks,   composition  of, 

212. 
Polypterus  (many-finned),  296. 
Porphyry,  213. 
Potholes,  26,  63. 
Primordial  beach,  269. 
Protohippus  (first  horse),  879. 
Protozoa  (first  and  lowest  living 

things),  266,  352. 
Psychozoic  (pertaining  to  rational 
life),  260. 
era,  407. 
Pteranodon      (winged-toothless), 

356. 
Pterichthys  (winged  fish),  292. 


424 


IJSDEX. 


Pterosaur  (winged  lizard),  837. 
Pumice,  214. 

Quaternary  period,  385. 

period  in  Eastern  North  Amer- 
ica, 386. 

period  in  Western  North  Amer- 
ica, 393. 

period,  life-system  of,  397. 

period,  mammals  of,  in  Amer- 
ica, 398. 

period,  mammals  of,  in  Aus- 
tralia, 402. 

period,  mammals  of,  in  Eu- 
rope, 397. 

pe»riod,  mammals  of,  in  South 
America,  401. 

period,  subdivisions  of,  385. 

Rafts,  88. 

Rain  and  rivers,  erosive  action  of, 
18. 

final  effect,  28. 

rate  of  erosion  by,  19. 
Ramphorhynchus      (beak-snout), 

338. 
Range  of  species,  genera,etc.,  120. 
Ravines,  gorges,  canons,  23. 
Reefs  and  keys  of  Florida,  109. 

how  formed,  111. 
Reefs  of  Pacific,  97. 
Regions,  botanical,  119. 

definition  of,  120. 

primary,  128. 

primary,  subdivisions  of,  128. 

zoological,  122. 
Reptiles,  age  of,  324. 
Rhyolite,  214. 
River-beds  of  'California,  old,  395. 

as  indicators  of  crust-move- 
ments, 28,  170. 

deltas,  subsidence  of,  169. 

drift-man  in  America,  414. 
Rivers,  deposits  at  the  mouths  of, 
38. 

deposits  of  old,  392. 

erosive  action  of,  30. 

flood-plain  deposits  of,  30. 

winding  course  of,  29. 
Rock  disintegration,  rate  of,  13. 

disintegration,  explanation  of, 
14. 
Rocks,  classes  of,  178. 


Rocks,  defined,  178. 

igneous,  210. 

meLamorphic,  224. 

stratified,  cause  of  consolida- 
tion of,  182. 

stratified,  classification  of,  205, 

stratified,  description  of,  179, 

stratified,  extent  of,  180. 

stratified,  origin  of,  181. 

stratified,  principal  kinds  of, 
180. 

structures  common  to  all,  228. 

unstratified,  210. 

St.  Anthony,  Falls  of,  22. 
Saline  lakes,  how  formed,  76. 

lakes,  chemical  deposits  in,  76. 
Salt  lakes,  liow  formed,  77. 
Sauropus  (lizard-foot),  321. 
Scaphites  (stone-boat),  353. 
Sea-beaches,  elevation  of,  391. 
Section  of  earth-crust,  ideal,  261. 
Sediments,     transportation     and 

distribution  of,  26. 
Sequoia  :   genus  of  conifers,   in- 
cluding  Redwoods  and   Big 
Trees  of  California,  368. 
Sertularia  (a  little  garland),  276. 
Sharks,  291,  318,  354,  873. 
Shell  limestone,  how  formed,  114. 

mounds,  412. 
Sierra  Nevada  range,  formation 

of,  348. 
Sigillaria  (from   sigillum,  a  seal), 

308. 
Sigillarid,  287,  308. 
Silica,  deposits  of,  76. 
Silurian  animals,  275. 

rocks,   area  of,  in  the  United 

States,  273. 
system,  271. 

times,  life-system  of,  273. 
times,  plants  of,  274. 
times,  subdivisions  of,  273. 
times,  physical   geography  of, 
273. 
Slaty  cleavage,  194. 
Snow-line,  58. 
Soil,  depth  of,  12. 

origin  of,  10. 
Solfataras  (hot  sulphur  springs), 

145. 
Sorting  power  of  water,  27. 


INDEX. 


425 


Species,    geographical    distribu- 
tion of,  118. 
origin  of  geographical  diversity 
of,  130. 
Sphenothallus      (wedge     frond), 

274. 
Springs,  69. 
great,  69. 
mineral,  71. 
Sqiialodont    (shark  -  toothed  :     a 

family  of  true  sharks),  818. 
Stalactites  and  stalagmites,  72. 
Stegosaur  (covered  lizard),  346. 
Strata,  crumpling  of,  185. 

folding  of,  185. 
Stratification  explained,  27. 
Stratified  rocks,  classification  of, 
205. 
rocks,  divisions    and    subdivi- 
sions of,  207. 
rocks,  how  relative  age  of,  is 
determined,  205. 
Strike  and  dip  defined,  187. 
Strombodes      pentagonus      (five- 
angled     strombus-like     ani- 
mal), 275. 
Structural  geology,  173. 
Sub-carboniferous,  297. 
Submarine  banks,  41. 
Subsidence    of     crust,    gradual, 
169. 
of  Pacific  bottom,  amount  of, 

107. 
of  Pacific  bottom,  time  of,  107. 
of  river  deltas,  16P. 
Succinifer  (amber-bearing),  371. 
Sulphur,  deposits  of,  76. 
Swjimp,  Great  Dismal,  86. 
Syenite,  212. 

Syncline  and    anticline   defined. 
188. 

Table-mountains,  247. 
Tachylite,  214. 

Taxodium,  bald  cypress  of  South- 
ern swamps,  368. 
Teleost  (complete  bone),  291. 
Terraces,  392. 
Tertiary  period,  364. 

period,  animals  of,  370. 

period,  coal  of,  366. 

period,  crust-movements  during 
and  closing,  384. 


Tei-tiary  period,  lake  deposits  ofj 
377. 

period,  life-system  of,  368. 

period,  mammals  of,  375. 

period,  physical  geography  of, 
366. 

period,  plants  of,  368. 

period,  subdivisions  of,  364. 

system,  areas  of,  865. 
Theromorpha  (beast-like),  328. 
Tides  and  waves,  agency  of,  41. 

and  waves,  effect  on  coast-line, 
42. 
Toxoceras  (bow-horn),  354. 
Trachyte,  139,  214. 
Transporting  power  of  water,  26. 
Trappean  rocks,  213. 
Triceratops   (three-horned    face), 

362. 
Triassic  period,  325. 

period,  animals  of,  326. 

period,  life-system  of,  325. 

period,  plants  of,  826. 
Trigonia  (three-angled),  331. 
Trilobite  (three-lobed  stone),  283, 
Tufa,  139,  223. 
Turrulite  (stone  tower),  354. 

Unconformity,  190. 
Unstratified  rocks,  210. 

Vegetable  accumulations.  84 
Veins,  age  of,  230. 

contents  of,  234. 

fissure,  284. 

irregularities  of,  236. 

metalliferous,  234. 

mineral,  233. 

origin  of,  237. 

structure  of,  235. 

surface  changes  of,  237. 
Volcanic  cinders,  ashes,  etc.,  135 

dikes,  141. 

eruptions,  cause  of,  144. 

eruptions,  phenomena  of,  135. 

gases  and  vapors,  139. 

phenomena,  secondary,  145. 

rocks,  214. 

rocks,  age  of,  219. 

rocks,  different  kinds  of.  215. 

rocks,  intercalary  beds  of.  219. 

rocks,  modes  of  eruption,  215. 

rocks,  modes  of  occurrence,  216. 


426 


INDEX, 


Volcanic  rocks,  sub-groups  of,  215. 
Volcanoes,  133. 

age  of,  143. 

erupted  matters  of,  136. 

mode  of  formation  of,  140. 

number,  size,  and  distribution 
of,  134. 

two  types  of,  135. 

Water,  agencies  of,  17. 
chemical  agency  of,  67. 
mechanical  agency  of,  18. 
perpetual  ground,  68. 
sorting  power  of,  27. 
transporting  power  of,  26, 


Water,  underground,  67. 

volcanic,  68. 
Waterfalls,  recession  of,  2Q 
Waves,  land  formed  by,  50. 

nature  of  deposits  by,  46. 

and  tides,  agency  of,  41. 

transportation  and  deposit  bV) 
46. 
Wells,  artesian,  70. 
Winds,  action  of,  16. 

Yosemite  Falls,  28. 

Zaphrentif?  (proper  name),  288, 
Zoological  regions,  122,  188. 


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